xfs: move setting bt_logical_sectorsize out of xfs_setsize_buftarg

bt_logical_sectorsize and the associated mask is set based on the
constant logical block size in the block_device structure and thus
doesn't need to be updated in xfs_setsize_buftarg.  Move it into
xfs_alloc_buftarg so that it is only done once per buftarg.

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