mempool: uninline function to check cookies

mempool: uninline function to check cookies

There's no reason to keep this function inlined. Move it to
rte_mempool.c. We need to export the function for when compiling
with shared libraries + debug. We also need to keep the macro,
because we don't want to call an empty function when debug is
disabled.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>