xfs: fix rtalloc rotoring when delalloc is in use

If we're trying to allocate real space for a delalloc reservation at
offset 0, we should use the rotor to spread files across the rt volume.

Switch the rtalloc to use the XFS_ALLOC_INITIAL_USER_DATA flag that
is set for any write at startoff to make it match the behavior for
the main data device.

Based on a patch from Darrick J. Wong.

Fixes: 6a94b1ac ("xfs: reinstate delalloc for RT inodes (if sb_rextsize == 1)")
Reported-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>