mbuf: use C11 atomic builtins for refcnt

mbuf: use C11 atomic builtins for refcnt

Use C11 atomic builtins with explicit ordering instead of rte_atomic
ops which enforce unnecessary barriers on aarch64.

Suggested-by: Olivier Matz <olivier.matz@6wind.com>
Suggested-by: Dodji Seketeli <dodji@redhat.com>
Signed-off-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>