xfs: release bli from transaction properly on fs shutdown

xfs: release bli from transaction properly on fs shutdown

If a filesystem shutdown occurs with a buffer log item in the CIL
and a log force occurs, the ->iop_unpin() handler is generally
expected to tear down the bli properly. This entails freeing the bli
memory and releasing the associated hold on the buffer so it can be
released and the filesystem unmounted.

If this sequence occurs while ->bli_refcount is elevated (i.e.,
another transaction is open and attempting to modify the buffer),
however, ->iop_unpin() may not be responsible for releasing the bli.
Instead, the transaction may release the final ->bli_refcount
reference and thus xfs_trans_brelse() is responsible for tearing
down the bli.

While xfs_trans_brelse() does drop the reference count, it only
attempts to release the bli if it is clean (i.e., not in the
CIL/AIL). If the filesystem is shutdown and the bli is sitting dirty
in the CIL as noted above, this ends up skipping the last
opportunity to release the bli. In turn, this leaves the hold on the
buffer and causes an unmount hang. This can be reproduced by running
generic/388 in repetition.

Update xfs_trans_brelse() to handle this shutdown corner case
correctly. If the final bli reference is dropped and the filesystem
is shutdown, remove the bli from the AIL (if necessary) and release
the bli to drop the buffer hold and ensure an unmount does not hang.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>