Btrfs: fix balancing oops when invalidate_inode_pages2 returns EBUSY

invalidate_inode_pages2_range may return -EBUSY occasionally
which results Oops. This patch fixes the issue by moving
invalidate_inode_pages2_range into a loop and keeping calling
it until the return value is not -EBUSY.

The EBUSY return is temporary, and can happen when the btrfs release page
function is unable to release a page because the EXTENT_LOCK
bit is set.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/relocation.c

@@ -2553,8 +2553,13 @@ int relocate_inode_pages(struct inode *inode, u64 start, u64 len)
 	last_index = (start + len - 1) >> PAGE_CACHE_SHIFT;
 
 	/* make sure the dirty trick played by the caller work */
+	while (1) {
 		ret = invalidate_inode_pages2_range(inode->i_mapping,
 						    first_index, last_index);
+		if (ret != -EBUSY)
+			break;
+		schedule_timeout(HZ/10);
+	}
 	if (ret)
 		goto out_unlock;