xfs: fix null bno_hint handling in xfs_rtallocate_rtg

xfs_bmap_rtalloc initializes the bno_hint variable to NULLRTBLOCK (aka
NULLFSBLOCK).  If the allocation request is for a file range that's
adjacent to an existing mapping, it will then change bno_hint to the
blkno hint in the bmalloca structure.

In other words, bno_hint is either a rt block number, or it's all 1s.
Unfortunately, commit ec12f97 didn't take the NULLRTBLOCK state
into account, which means that it tries to translate that into a
realtime extent number.  We then end up with an obnoxiously high rtx
number and pointlessly feed that to the near allocator.  This often
fails and falls back to the by-size allocator.  Seeing as we had no
locality hint anyway, this is a waste of time.

Fix the code to detect a lack of bno_hint correctly.  This was detected
by running xfs/009 with metadir enabled and a 28k rt extent size.

Cc: <stable@vger.kernel.org> # v6.12
Fixes: ec12f97 ("xfs: make the rtalloc start hint a xfs_rtblock_t")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Author: Darrick J. Wong

fs/xfs/xfs_rtalloc.c

@@ -1827,7 +1827,7 @@ xfs_rtallocate_rtg(
 	 * For an allocation to an empty file at offset 0, pick an extent that
 	 * will space things out in the rt area.
 	 */
-	if (bno_hint)
+	if (bno_hint != NULLFSBLOCK)
 		start = xfs_rtb_to_rtx(args.mp, bno_hint);
 	else if (!xfs_has_rtgroups(args.mp) && initial_user_data)
 		start = xfs_rtpick_extent(args.rtg, tp, maxlen);