xfs: make the seq argument to xfs_bmapi_convert_delalloc() optional

Allow callers to pass a NULLL seq argument if they don't care about
the fork sequence number.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>