xfs: align initial file allocations correctly

The function xfs_bmap_isaeof() is used to indicate that an
allocation is occurring at or past the end of file, and as such
should be aligned to the underlying storage geometry if possible.

Commit 27a3f8f ("xfs: introduce xfs_bmap_last_extent") changed the
behaviour of this function for empty files - it turned off
allocation alignment for this case accidentally. Hence large initial
allocations from direct IO are not getting correctly aligned to the
underlying geometry, and that is cause write performance to drop in
alignment sensitive configurations.

Fix it by considering allocation into empty files as requiring
aligned allocation again.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_bmap.c

@@ -1635,7 +1635,7 @@ xfs_bmap_last_extent(
  * blocks at the end of the file which do not start at the previous data block,
  * we will try to align the new blocks at stripe unit boundaries.
  *
- * Returns 0 in bma->aeof if the file (fork) is empty as any new write will be
+ * Returns 1 in bma->aeof if the file (fork) is empty as any new write will be
  * at, or past the EOF.
  */
 STATIC int
@@ -1650,9 +1650,14 @@ xfs_bmap_isaeof(
 	bma->aeof = 0;
 	error = xfs_bmap_last_extent(NULL, bma->ip, whichfork, &rec,
 				     &is_empty);
-	if (error || is_empty)
+	if (error)
 		return error;
 
+	if (is_empty) {
+		bma->aeof = 1;
+		return 0;
+	}
+
 	/*
 	 * Check if we are allocation or past the last extent, or at least into
 	 * the last delayed allocated extent.