chunkfs: use mutex for continuations lock instead of spin lock

This was because we acquire rwsem inside of section,
where spin lock is acquired.

Must fix next issue:

<3>[  514.264114] BUG: sleeping function called from invalid context at kernel/locking/rwsem.c:20
<3>[  514.268201] in_atomic(): 1, irqs_disabled(): 0, pid: 2052, name: ls
<4>[  514.269031] CPU: 0 PID: 2052 Comm: ls Tainted: G      D W    3.13.0-rc3+ #90
<4>[  514.269956] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
<4>[  514.270977]  0000000000000000 ffff88007a282920 ffffffff813c257e ffff88007ca2a0d0
<4>[  514.272336]  ffff88007a282940 ffffffff8105dff4 ffff88007b29cc00 ffff88007a282a14
<4>[  514.273680]  ffff88007a282958 ffffffff813c5f08 ffff88007b29cc58 ffff88007a2829b0
<4>[  514.275115] Call Trace:
<4>[  514.275530]  [<ffffffff813c257e>] dump_stack+0x4e/0x7a
<4>[  514.276070]  [<ffffffff8105dff4>] __might_sleep+0xf1/0xf6
<4>[  514.276578]  [<ffffffff813c5f08>] down_read+0x1b/0x2c
<4>[  514.277095]  [<ffffffffa011ff82>] ext4_xattr_get+0x37/0x13e [ext4]
<4>[  514.277632]  [<ffffffffa0120dc8>] ext4_xattr_user_get+0x31/0x3f [ext4]
<4>[  514.278196]  [<ffffffff81140cb9>] generic_getxattr+0x47/0x59
<4>[  514.278714]  [<ffffffff81184555>] get_set_cont_data+0x82/0xfe
<4>[  514.279261]  [<ffffffff8111a00a>] ? ____cache_alloc+0x1c/0x284
<4>[  514.279895]  [<ffffffff8118481a>] ? load_continuation.isra.3+0x48/0xc0
<4>[  514.280394]  [<ffffffff811846e2>] get_cont_data+0x27/0xcc
<4>[  514.280624]  [<ffffffff81184867>] load_continuation.isra.3+0x95/0xc0
<4>[  514.281181]  [<ffffffff811849c3>] chunkfs_get_next_cont+0x115/0x157
<4>[  514.281780]  [<ffffffff810f4492>] ? __inc_zone_state+0x5e/0x6c
<4>[  514.282300]  [<ffffffff810f4709>] ? zone_statistics+0x77/0x7e
<4>[  514.282824]  [<ffffffff811f63bc>] ? put_dec+0x52/0x57
<4>[  514.283319]  [<ffffffff811f6bd4>] ? number.isra.1+0x13f/0x25b
<4>[  514.284761]  [<ffffffff811f6bd4>] ? number.isra.1+0x13f/0x25b
<4>[  514.285313]  [<ffffffff811f8433>] ? vsnprintf+0x82/0x3f7
<4>[  514.285860]  [<ffffffff810643d6>] ? __dequeue_entity+0x2e/0x33
<4>[  514.286397]  [<ffffffff81001618>] ? __switch_to+0x1c1/0x3e8
<4>[  514.286909]  [<ffffffff813c6ac0>] ? _raw_spin_unlock_irq+0x13/0x26
<4>[  514.287472]  [<ffffffff8105e747>] ? finish_task_switch+0x48/0xba
<4>[  514.288568]  [<ffffffff813c442b>] ? __schedule+0x3fc/0x57b
<4>[  514.289319]  [<ffffffff813c48e3>] ? preempt_schedule_irq+0x3b/0x56
<4>[  514.290149]  [<ffffffff813c7290>] ? retint_kernel+0x20/0x30
<4>[  514.290729]  [<ffffffff810765c7>] ? vprintk_emit+0x3be/0x3e4
<4>[  514.291317]  [<ffffffff813c02c5>] ? printk+0x62/0x64
<4>[  514.291854]  [<ffffffff81184a60>] chunkfs_get_cont_at_offset+0x5b/0xf0
<4>[  514.292458]  [<ffffffff811836d5>] chunkfs_open_cont_file+0x5b/0xe1
<4>[  514.292734]  [<ffffffff8118380e>] chunkfs_open+0x3f/0x7b
<4>[  514.293314]  [<ffffffff811837cf>] ? chunkfs_close_cont_file+0x74/0x74
<4>[  514.293923]  [<ffffffff81124855>] do_dentry_open+0x214/0x232
<4>[  514.294511]  [<ffffffff811248a1>] finish_open+0x2e/0x38
<4>[  514.295061]  [<ffffffff81130da5>] do_last+0x84c/0x9e4
<4>[  514.295647]  [<ffffffff8113116f>] path_openat+0x232/0x4ec
<4>[  514.296215]  [<ffffffff81132469>] do_filp_open+0x35/0x7a
<4>[  514.296763]  [<ffffffff813c6a73>] ? _raw_spin_unlock+0x12/0x25
<4>[  514.297331]  [<ffffffff8113bf08>] ? __alloc_fd+0xe2/0xf2
<4>[  514.297887]  [<ffffffff81124f6a>] do_sys_open+0x6b/0xfa
<4>[  514.298431]  [<ffffffff81125023>] SyS_openat+0xf/0x11
<4>[  514.298946]  [<ffffffff813cc7e2>] system_call_fastpath+0x16/0x1b