xfs: remove NULLEXTNUM

We only ever set a field to this constant for an impossible to reach
error case in xfs_bmap_search_extents.  That functions has been removed,
so we can remove the constant as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>


diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
index dcc847c..c2e13db 100644
--- a/fs/xfs/libxfs/xfs_bmap.c
+++ b/fs/xfs/libxfs/xfs_bmap.c
@@ -4260,7 +4260,7 @@ xfs_bmapi_allocate(
 	if (bma->wasdel) {
 		bma->length = (xfs_extlen_t)bma->got.br_blockcount;
 		bma->offset = bma->got.br_startoff;
-		if (bma->idx != NULLEXTNUM && bma->idx) {
+		if (bma->idx) {
 			xfs_bmbt_get_all(xfs_iext_get_ext(ifp, bma->idx - 1),
 					 &bma->prev);
 		}