xfs: factor out AG header initialisation from growfs core

The intialisation of new AG headers is mostly common with the
userspace mkfs code and growfs in the kernel, so start factoring it
out so we can move it to libxfs and use it in both places.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>