xfs: separate out setting buftarg atomic writes limits

Separate out setting buftarg atomic writes limits into a dedicated
function, xfs_configure_buftarg_atomic_writes(), to keep the specific
functionality self-contained.

For naming consistency, rename xfs_setsize_buftarg() ->
xfs_configure_buftarg().

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
[jpg: separate out from patch "xfs: ignore HW which ..."]
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>