xfs: mark xfs_buf_free as might_sleep()

xfs_buf_free can call vunmap, which can sleep.  The vunmap path is an
unlikely one, so add might_sleep to ensure calling xfs_buf_free from
atomic context gets caught more easily.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Christoph Hellwig

fs/xfs/xfs_buf.c

@@ -105,6 +105,7 @@ xfs_buf_free(
 {
 	unsigned int		size = BBTOB(bp->b_length);
 
+	might_sleep();
 	trace_xfs_buf_free(bp, _RET_IP_);
 
 	ASSERT(list_empty(&bp->b_lru));