env: define mempool/memzone name length limits

DPDK memzone name limit is 32 and mempool's 29 as it
internally reserves a memzone with "MP_" prefix.

DPDK also offers defines for those limits, but we
obviously can't use them in generic env header file.

Change-Id: I16abcf404eee390a9033135e996cba1716baca5f
Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
Reviewed-on: https://review.gerrithub.io/416979


Tested-by: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>