xfs: map unwritten blocks in XFS_IOC_{ALLOC,FREE}SP just like fallocate

The old ALLOCSP/FREESP ioctls in XFS can be used to preallocate space at
the end of files, just like fallocate and RESVSP.  Make the behavior
consistent with the other ioctls.

Reported-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>

fs/xfs/xfs_ioctl.c

@@ -687,7 +687,8 @@ xfs_ioc_space(
 
 	if (bf->l_start > XFS_ISIZE(ip)) {
 		error = xfs_alloc_file_space(ip, XFS_ISIZE(ip),
-				bf->l_start - XFS_ISIZE(ip), 0);
+				bf->l_start - XFS_ISIZE(ip),
+				XFS_BMAPI_PREALLOC);
 		if (error)
 			goto out_unlock;
 	}