xfs: hoist out xfs_resizefs_init_new_ags()

Move out related logic for initializing new added AGs to a new helper
in preparation for shrinking. No logic changes.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>