xfs: revert 1baa2800 ("xfs: remove the unused XFS_ALLOC_USERDATA flag")

Revert this commit, as it caused periodic regressions in xfs/173 w/
1k blocks.

[1] https://lore.kernel.org/lkml/20190919014602.GN15734@shao2-debian/Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>

fs/xfs/libxfs/xfs_alloc.h

@@ -81,9 +81,10 @@ typedef struct xfs_alloc_arg {
 /*
  * Defines for datatype
  */
-#define XFS_ALLOC_INITIAL_USER_DATA	(1 << 0)/* special case start of file */
-#define XFS_ALLOC_USERDATA_ZERO		(1 << 1)/* zero extent on allocation */
-#define XFS_ALLOC_NOBUSY		(1 << 2)/* Busy extents not allowed */
+#define XFS_ALLOC_USERDATA		(1 << 0)/* allocation is for user data*/
+#define XFS_ALLOC_INITIAL_USER_DATA	(1 << 1)/* special case start of file */
+#define XFS_ALLOC_USERDATA_ZERO		(1 << 2)/* zero extent on allocation */
+#define XFS_ALLOC_NOBUSY		(1 << 3)/* Busy extents not allowed */
 
 static inline bool
 xfs_alloc_is_userdata(int datatype)

fs/xfs/libxfs/xfs_bmap.c

@@ -4042,8 +4042,12 @@ xfs_bmapi_allocate(
 	 */
 	if (!(bma->flags & XFS_BMAPI_METADATA)) {
 		bma->datatype = XFS_ALLOC_NOBUSY;
-		if (whichfork == XFS_DATA_FORK && bma->offset == 0)
+		if (whichfork == XFS_DATA_FORK) {
+			if (bma->offset == 0)
 				bma->datatype |= XFS_ALLOC_INITIAL_USER_DATA;
+			else
+				bma->datatype |= XFS_ALLOC_USERDATA;
+		}
 		if (bma->flags & XFS_BMAPI_ZERO)
 			bma->datatype |= XFS_ALLOC_USERDATA_ZERO;
 	}