xfs: get allocation alignment from the buftarg

Needed to feed into the allocation routine to guarantee the memory
buffers we add to bios are correctly aligned to the underlying
device.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>