xfs: move xfs_bmapi_reserve_delalloc to xfs_iomap.c

xfs: move xfs_bmapi_reserve_delalloc to xfs_iomap.c

Source kernel commit: 7c879c8275c0505c551f0fc6c152299c8d11f756

Delalloc reservations are not supported in userspace, and thus it doesn't
make sense to share this helper with xfsprogs.c.  Move it to xfs_iomap.c
toward the two callers.

Note that there rest of the delalloc handling should probably eventually
also move out of xfs_bmap.c, but that will require a bit more surgery.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>