With GCC 7 we need to explicitly document when we are falling through from
one switch case to another.
Fixes: af75078fece3 ("first public release")
Fixes: 8bae1da2afe0 ("hash: fallback to software CRC32 implementation")
Fixes: 9ec201f5d6e7 ("mbuf: provide bulk allocation")
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>