xfs: convert mount flags to features

Replace m_flags feature checks with xfs_has_<feature>() calls and
rework the setup code to set flags in m_features.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>