xfs: fix a missing unlock on error in xfs_fs_map_blocks

We also need to drop the iolock when invalidate_inode_pages2 fails, not
only on all other error or successful cases.

Fixes: 52785112 ("xfs: implement pNFS export operations")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>