xfs: lift a xfs_valid_startblock into xfs_bmapi_allocate

xfs_bmapi_convert_delalloc has a xfs_valid_startblock check on the block
allocated by xfs_bmapi_allocate.  Lift it into xfs_bmapi_allocate as
we should assert the same for xfs_bmapi_write.

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>