dma: mv_xor: do not sync the DMA buffer after being deallocated

DMA_CHECK was unhappy:
mv_xor mv_xor.0: DMA-API: device driver tries to sync DMA memory it has not

Also, the xor test was causing the mv_xor_slot_cleanup() to incorrectly
dealocate the destination buffer. This is fixed as well, since the deallocation
is done in mv_xor_probe() (with correct flags) now:
mv_xor mv_xor.0: DMA-API: device driver frees DMA memory with different direction [device address=0x000000001dea4000] [size=4096 bytes] [mapped with DMA_FROM_DEVICE] [unmapped with DMA_BIDIRECTIONAL]

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
dma: mv_xor: remove minimal offload length threshold

This fatally confuses dma_async_memcpy_buf_to_pg which thinks it signals an out
of memory condition and retries indefinitelly, causing a soft lockup. The
threshold does not seem to be enforced by hardware (couldn't find anything like
that in a datasheet) and things seems to work fine without it. If there's a
performance penalty, it probably should be dealt with in dmaengine.

The following crash (with too small skb structure being dma_memcpy()'d) is
reprodcibly triggered by an accept() quickly followed by a read():
http://v3.sk/~lkundrak/so-beautiful/crasher.pl

BUG: soft lockup - CPU#0 stuck for 23s! [perl:715]
Modules linked in:
Pid: 715, comm:                 perl
CPU: 0    Not tainted  (3.6.7-2.fc18.armv5tel.kirkwood #1)
PC is at mv_xor_prep_dma_memcpy+0x14/0x168
LR is at dma_async_memcpy_pg_to_pg+0x100/0x1dc
sp : df197ce0  ip : c023816c  fp : 00000000
r10: c09a6720  r9 : 0000012a  r8 : 115d7000
r7 : ddd4f57c  r6 : ddd4f57c  r5 : ddd4f4a8  r4 : 00000034
r3 : 00000034  r2 : 115d7000  r1 : 116b912a  r0 : ddd4f57c
Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
Control: 0005397f  Table: 1f218000  DAC: 00000015
[<c000edac>] (unwind_backtrace+0x0/0x124) from [<c0074ce4>] (watchdog_timer_fn+0xf0/0x144)
[<c0074ce4>] (watchdog_timer_fn+0xf0/0x144) from [<c003aa10>] (__run_hrtimer+0xb0/0x1d4)
[<c003aa10>] (__run_hrtimer+0xb0/0x1d4) from [<c003b224>] (hrtimer_interrupt+0x104/0x250)
[<c003b224>] (hrtimer_interrupt+0x104/0x250) from [<c0016024>] (orion_timer_interrupt+0x24/0x34)
[<c0016024>] (orion_timer_interrupt+0x24/0x34) from [<c0075470>] (handle_irq_event_percpu+0x38/0x23c)
[<c0075470>] (handle_irq_event_percpu+0x38/0x23c) from [<c00756a4>] (handle_irq_event+0x30/0x40)
[<c00756a4>] (handle_irq_event+0x30/0x40) from [<c0077cd4>] (handle_level_irq+0xcc/0xdc)
[<c0077cd4>] (handle_level_irq+0xcc/0xdc) from [<c0074e9c>] (generic_handle_irq+0x28/0x38)
[<c0074e9c>] (generic_handle_irq+0x28/0x38) from [<c0009b64>] (handle_IRQ+0x68/0x8c)
[<c0009b64>] (handle_IRQ+0x68/0x8c) from [<c0444794>] (__irq_svc+0x34/0x78)
[<c0444794>] (__irq_svc+0x34/0x78) from [<c0238180>] (mv_xor_prep_dma_memcpy+0x14/0x168)
[<c0238180>] (mv_xor_prep_dma_memcpy+0x14/0x168) from [<c0236010>] (dma_async_memcpy_pg_to_pg+0x100/0x1dc)
[<c0236010>] (dma_async_memcpy_pg_to_pg+0x100/0x1dc) from [<c0237594>] (dma_memcpy_pg_to_iovec+0xf0/0x180)
[<c0237594>] (dma_memcpy_pg_to_iovec+0xf0/0x180) from [<c03832f8>] (dma_skb_copy_datagram_iovec+0x100/0x1d4)
[<c03832f8>] (dma_skb_copy_datagram_iovec+0x100/0x1d4) from [<c03a9950>] (tcp_recvmsg+0x630/0xac0)
[<c03a9950>] (tcp_recvmsg+0x630/0xac0) from [<c03c8dd8>] (inet_recvmsg+0x48/0x5c)
[<c03c8dd8>] (inet_recvmsg+0x48/0x5c) from [<c035ba54>] (sock_aio_read+0x100/0x120)
[<c035ba54>] (sock_aio_read+0x100/0x120) from [<c00e53e0>] (do_sync_read+0x98/0xd4)
[<c00e53e0>] (do_sync_read+0x98/0xd4) from [<c00e5d54>] (vfs_read+0xb4/0x184)
[<c00e5d54>] (vfs_read+0xb4/0x184) from [<c00e5e60>] (sys_read+0x3c/0x70)
[<c00e5e60>] (sys_read+0x3c/0x70) from [<c0008c60>] (ret_fast_syscall+0x0/0x2c)
Kernel panic - not syncing: softlockup: hung tasks
[<c000edac>] (unwind_backtrace+0x0/0x124) from [<c043e0ac>] (panic+0x80/0x1e0)
[<c043e0ac>] (panic+0x80/0x1e0) from [<c0074d08>] (watchdog_timer_fn+0x114/0x144)
[<c0074d08>] (watchdog_timer_fn+0x114/0x144) from [<c003aa10>] (__run_hrtimer+0xb0/0x1d4)
[<c003aa10>] (__run_hrtimer+0xb0/0x1d4) from [<c003b224>] (hrtimer_interrupt+0x104/0x250)
[<c003b224>] (hrtimer_interrupt+0x104/0x250) from [<c0016024>] (orion_timer_interrupt+0x24/0x34)
[<c0016024>] (orion_timer_interrupt+0x24/0x34) from [<c0075470>] (handle_irq_event_percpu+0x38/0x23c)
[<c0075470>] (handle_irq_event_percpu+0x38/0x23c) from [<c00756a4>] (handle_irq_event+0x30/0x40)
[<c00756a4>] (handle_irq_event+0x30/0x40) from [<c0077cd4>] (handle_level_irq+0xcc/0xdc)
[<c0077cd4>] (handle_level_irq+0xcc/0xdc) from [<c0074e9c>] (generic_handle_irq+0x28/0x38)
[<c0074e9c>] (generic_handle_irq+0x28/0x38) from [<c0009b64>] (handle_IRQ+0x68/0x8c)
[<c0009b64>] (handle_IRQ+0x68/0x8c) from [<c0444794>] (__irq_svc+0x34/0x78)
[<c0444794>] (__irq_svc+0x34/0x78) from [<c0238180>] (mv_xor_prep_dma_memcpy+0x14/0x168)
[<c0238180>] (mv_xor_prep_dma_memcpy+0x14/0x168) from [<c0236010>] (dma_async_memcpy_pg_to_pg+0x100/0x1dc)
[<c0236010>] (dma_async_memcpy_pg_to_pg+0x100/0x1dc) from [<c0237594>] (dma_memcpy_pg_to_iovec+0xf0/0x180)
[<c0237594>] (dma_memcpy_pg_to_iovec+0xf0/0x180) from [<c03832f8>] (dma_skb_copy_datagram_iovec+0x100/0x1d4)
[<c03832f8>] (dma_skb_copy_datagram_iovec+0x100/0x1d4) from [<c03a9950>] (tcp_recvmsg+0x630/0xac0)
[<c03a9950>] (tcp_recvmsg+0x630/0xac0) from [<c03c8dd8>] (inet_recvmsg+0x48/0x5c)
[<c03c8dd8>] (inet_recvmsg+0x48/0x5c) from [<c035ba54>] (sock_aio_read+0x100/0x120)
[<c035ba54>] (sock_aio_read+0x100/0x120) from [<c00e53e0>] (do_sync_read+0x98/0xd4)
[<c00e53e0>] (do_sync_read+0x98/0xd4) from [<c00e5d54>] (vfs_read+0xb4/0x184)
[<c00e5d54>] (vfs_read+0xb4/0x184) from [<c00e5e60>] (sys_read+0x3c/0x70)
[<c00e5e60>] (sys_read+0x3c/0x70) from [<c0008c60>] (ret_fast_syscall+0x0/0x2c)
Rebooting in 1 seconds..

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
LR: dma: mv_xor: test for small transfers