mbuf: rework check on mbuf freeing
mbuf: rework check on mbuf freeing

Allow poll-mode drivers to maintain their own caches of mbufs, by allowing them
to check if it's ok to free an mbuf (to their local cache) without actually
freeing it back to the memory pool itself.

Signed-off-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>
Signed-off-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@6wind.com>

msune/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,tsphillips/dpdk-fork,tsphillips/dpdk-fork,msune/dpdk,tsphillips/dpdk-fork,msune/dpdk,john-mcnamara-intel/dpdk,venkynv/dpdk-mirror,mixja/dpdk,venkynv/dpdk-mirror,john-mcnamara-intel/dpdk,mixja/dpdk,venkynv/dpdk-mirror,tsphillips/dpdk-fork,mixja/dpdk,mixja/dpdk,msune/dpdk