xfs: unlock inodes when erroring out of xfs_trans_alloc_dir

In the Linux kernel, the following vulnerability has been resolved:

xfs: unlock inodes when erroring out of xfs_trans_alloc_dir

Debugging a filesystem patch with generic/475 caused the system to hang
after observing the following sequences in dmesg:

XFS (dm-0): metadata I/O error in "xfs_imap_to_bp+0x61/0xe0 [xfs]" at daddr 0x491520 len 32 error 5
XFS (dm-0): metadata I/O error in "xfs_btree_read_buf_block+0xba/0x160 [xfs]" at daddr 0x3445608 len 8 error 5
XFS (dm-0): metadata I/O error in "xfs_imap_to_bp+0x61/0xe0 [xfs]" at daddr 0x138e1c0 len 32 error 5
XFS (dm-0): log I/O error -5
XFS (dm-0): Metadata I/O Error (0x1) detected at xfs_trans_read_buf_map+0x1ea/0x4b0 [xfs] (fs/xfs/xfs_trans_buf.c:311). Shutting down filesystem.
XFS (dm-0): Please unmount the filesystem and rectify the problem(s)
XFS (dm-0): Internal error dqp->q_ino.reserved < dqp->q_ino.count at line 869 of file fs/xfs/xfs_trans_dquot.c. Caller xfs_trans_dqresv+0x236/0x440 [xfs]
XFS (dm-0): Corruption detected. Unmount and run xfs_repair
XFS (dm-0): Unmounting Filesystem be6bcbcc-9921-4deb-8d16-7cc94e335fa7

The system is stuck in unmount trying to lock a couple of inodes so that
they can be purged. The dquot corruption notice above is a clue to what
happened -- a link() call tried to set up a transaction to link a child
into a directory. Quota reservation for the transaction failed after IO
errors shut down the filesystem, but then we forgot to unlock the inodes
on our way out. Fix that.