Btrfs: avoid triggering bug_on() when we fail to start inode caching task

When running stress test(including snapshots,balance,fstress), we trigger
the following BUG_ON() which is because we fail to start inode caching task.

[  181.131945] kernel BUG at fs/btrfs/inode-map.c:179!
[  181.137963] invalid opcode: 0000 [#1] SMP
[  181.217096] CPU: 11 PID: 2532 Comm: btrfs Not tainted 3.14.0 #1
[  181.240521] task: ffff88013b621b30 ti: ffff8800b6ada000 task.ti: ffff8800b6ada000
[  181.367506] Call Trace:
[  181.371107]  [<ffffffffa036c1be>] btrfs_return_ino+0x9e/0x110 [btrfs]
[  181.379191]  [<ffffffffa038082b>] btrfs_evict_inode+0x46b/0x4c0 [btrfs]
[  181.387464]  [<ffffffff810b5a70>] ? autoremove_wake_function+0x40/0x40
[  181.395642]  [<ffffffff811dc5fe>] evict+0x9e/0x190
[  181.401882]  [<ffffffff811dcde3>] iput+0xf3/0x180
[  181.408025]  [<ffffffffa03812de>] btrfs_orphan_cleanup+0x1ee/0x430 [btrfs]
[  181.416614]  [<ffffffffa03a6abd>] btrfs_mksubvol.isra.29+0x3bd/0x450 [btrfs]
[  181.425399]  [<ffffffffa03a6cd6>] btrfs_ioctl_snap_create_transid+0x186/0x190 [btrfs]
[  181.435059]  [<ffffffffa03a6e3b>] btrfs_ioctl_snap_create_v2+0xeb/0x130 [btrfs]
[  181.444148]  [<ffffffffa03a9656>] btrfs_ioctl+0xf76/0x2b90 [btrfs]
[  181.451971]  [<ffffffff8117e565>] ? handle_mm_fault+0x475/0xe80
[  181.459509]  [<ffffffff8167ba0c>] ? __do_page_fault+0x1ec/0x520
[  181.467046]  [<ffffffff81185b35>] ? do_mmap_pgoff+0x2f5/0x3c0
[  181.474393]  [<ffffffff811d4da8>] do_vfs_ioctl+0x2d8/0x4b0
[  181.481450]  [<ffffffff811d5001>] SyS_ioctl+0x81/0xa0
[  181.488021]  [<ffffffff81680b69>] system_call_fastpath+0x16/0x1b

We should avoid triggering BUG_ON() here, instead, we output warning messages
and clear inode_cache option.

Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>