Btrfs: don't release invalid page in btrfs_page_exists_in_range()

In inode.c:btrfs_page_exists_in_range(), if the page we got from
the radix tree is an exception entry, which can't be retried, we
exit the loop with a non-NULL page and then call page_cache_release
against it, which is not ok since it's not a valid page. This could
also make us return true when we shouldn't.
Signed-off-by: NFilipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/inode.c

@@ -6776,6 +6776,7 @@ bool btrfs_page_exists_in_range(struct inode *inode, loff_t start, loff_t end)
 			 * here as an exceptional entry: so return it without
 			 * attempting to raise page count.
 			 */
+			page = NULL;
 			break; /* TODO: Is this relevant for this use case? */
 		}