eal/bsd: fix build

When compiling on FreeBSD, lots of warnings/errors are thrown for
unused parameter. Fix these by marking the parameters as unused
in the code.

Fixes: 1009ba1704f9 ("mem: add internal API to get and set segment fd")
Fixes: 3a44687139eb ("mem: allow querying offset into segment fd")

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>