mbuf: fix reference counter integer promotion

mbuf: fix reference counter integer promotion

GCC 8.1 warned:

"1 + value", where value is an uint16_t causes promotion
to a signed int.  The compiler complained that we are
shoving an int into a uint16_t return type with different
size and sign.

Bumping and returning value directly instead removes the
promotion and the problem.

Fixes: f20b50b946da ("mbuf: optimize refcnt update")
Fixes: a53aa2b9f3be ("mbuf: support attaching external buffer")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>