xfs: make is_log_ag() a first class helper

We check if an ag contains the log in many places, so make this
a first class XFS helper by lifting it to fs/xfs/libxfs/xfs_ag.h and
renaming it xfs_ag_contains_log(). The convert all the places that
check if the AG contains the log to use this helper.

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