dm cache: add test_small_file_io_shrinker

Currently requires a large config: 8GB metadata, 40GB cache, 1TB origin
Setting mem=3G I was able to reproduce Ben England's reported crash
using a RHEL7 kernel:

[  659.079676] XFS (dm-8): Mounting Filesystem
[  659.213668] XFS (dm-8): Ending clean mount
[  890.929456] BUG: unable to handle kernel paging request at 0000000200007fff
[  890.937247] IP: [<0000000200007fff>] 0x200007ffe
[  890.942417] PGD 0
[  890.944667] Oops: 0010 [#1] SMP
[  890.948283] Modules linked in: xfs dm_cache_mq dm_cache() dm_bio_prison dm_persistent_data dm_bufio libcrc32c ebtable_nat ebtables ipt_MASQUERADE iptable_nat nf_nat_ipv4 nf_nat xt_CHECKSUM iptable_mangle bridge bnx2fc 8021q fcoe mrp garp libfcoe stp libfc llc scsi_tran
sport_fc scsi_tgt sunrpc ipt_REJECT nf_conntrack_ipv4 nf_defrag_ipv4 iptable_filter ip_tables ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 xt_state nf_conntrack ip6table_filter ip6_tables be2iscsi iscsi_boot_sysfs bnx2i cnic uio cxgb4i cxgb4 cxgb3i libcxgbi cxgb3 ib_iser
rdma_cm ib_cm iw_cm ib_sa ib_mad ib_core ib_addr iscsi_tcp libiscsi_tcp libiscsi scsi_transport_iscsi binfmt_misc vhost_net macvtap macvlan tun kvm_intel kvm iTCO_wdt iTCO_vendor_support ipmi_si ipmi_msghandler microcode pcspkr i2c_i801 lpc_ich mfd_core igb i7core_edac ed
ac_core iomemory_vsl(POF) ixgbe dca ptp pps_core mdio ses enclosure sg acpi_power_meter acpi_cpufreq mperf ext4 jbd2 mbcache sr_mod cdrom pata_acpi ata_generic ata_piix libata sd_mod crc_t10dif crct10dif_common mgag200 ttm drm_kms_helper drm i2c_algo_bit sysimgblt sysfill
rect i2c_core syscopyarea dm_mirror dm_region_hash dm_log dm_mod megaraid_sas
[  891.062550] CPU: 9 PID: 101 Comm: kswapd0 Tainted: PF       W IO-------------- T 3.10.0-73.el7.thinp_bpa.x86_64 #1
[  891.074095] Hardware name: FUJITSU                          PRIMERGY RX300 S6             /D2619, BIOS 6.00 Rev. 1.10.2619.N1           05/24/2011
[  891.088739] task: ffff8800b66a0000 ti: ffff8800b66a8000 task.ti: ffff8800b66a8000
[  891.097085] RIP: 0010:[<0000000200007fff>]  [<0000000200007fff>] 0x200007ffe
[  891.104958] RSP: 0018:ffff8800b66a9cc8  EFLAGS: 00010206
[  891.110882] RAX: 0000000200007fff RBX: 0000000000000080 RCX: 0000000000000000
[  891.118841] RDX: 0000000000001b5e RSI: ffff8800b66a9db0 RDI: ffff8800b9880cd8
[  891.126797] RBP: ffff8800b66a9d60 R08: 0000000000000000 R09: 0000000000000020
[  891.134754] R10: 00000000000000e8 R11: 0000000000000000 R12: 0000000000000001
[  891.142712] R13: ffff8800b9880cd8 R14: ffff8800b66a9db0 R15: 0000000000000000
[  891.150670] FS:  0000000000000000(0000) GS:ffff8800bf520000(0000) knlGS:0000000000000000
[  891.159694] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  891.166100] CR2: 0000000200007fff CR3: 00000000018a8000 CR4: 00000000000007e0
[  891.174057] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  891.182016] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  891.189976] Stack:
[  891.192218]  ffffffff81145755 ffffffff815c0cb2 ffff8800bb84b078 ffff8800b66a9d10
[  891.200503]  ffffffff81198f31 ffff8800b66a9dc0 000000000006c356 0000000000000164
[  891.208796]  ffff88003783e410 0000000000000001 000000000006c355 0000000000000059
[  891.217085] Call Trace:
[  891.219815]  [<ffffffff81145755>] ? shrink_slab+0xa5/0x330
[  891.225935]  [<ffffffff815c0cb2>] ? mutex_lock+0x12/0x2f
[  891.231863]  [<ffffffff81198f31>] ? vmpressure+0x61/0x90
[  891.237788]  [<ffffffff811492f6>] balance_pgdat+0x486/0x5c0
[  891.244005]  [<ffffffff81149583>] kswapd+0x153/0x400
[  891.249542]  [<ffffffff8107fab0>] ? wake_up_bit+0x30/0x30
[  891.255562]  [<ffffffff81149430>] ? balance_pgdat+0x5c0/0x5c0
[  891.261970]  [<ffffffff8107eb90>] kthread+0xc0/0xd0
[  891.267409]  [<ffffffff8107ead0>] ? kthread_create_on_node+0x110/0x110
[  891.274776]  [<ffffffff815cd06c>] ret_from_fork+0x7c/0xb0
[  891.280799]  [<ffffffff8107ead0>] ? kthread_create_on_node+0x110/0x110
[  891.288078] Code:  Bad RIP value.
[  891.291791] RIP  [<0000000200007fff>] 0x200007ffe
[  891.297053]  RSP <ffff8800b66a9cc8>
[  891.300939] CR2: 0000000200007fff