xfs: support lowmode allocations in xfs_bmap_exact_minlen_extent_alloc

Currently the debug-only xfs_bmap_exact_minlen_extent_alloc allocation
variant fails to drop into the lowmode last resort allocator, and
thus can sometimes fail allocations for which the caller has a
transaction block reservation.

Fix this by using xfs_bmap_btalloc_low_space to do the actual allocation.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/libxfs/xfs_bmap.c

@@ -3501,7 +3501,13 @@ xfs_bmap_exact_minlen_extent_alloc(
 	 */
 	ap->blkno = XFS_AGB_TO_FSB(ap->ip->i_mount, 0, 0);
 
-	return xfs_alloc_vextent_first_ag(args, ap->blkno);
+	/*
+	 * Call xfs_bmap_btalloc_low_space here as it first does a "normal" AG
+	 * iteration and then drops args->total to args->minlen, which might be
+	 * required to find an allocation for the transaction reservation when
+	 * the file system is very full.
+	 */
+	return xfs_bmap_btalloc_low_space(ap, args);
 }
 
 /*