core: get ride of pg_get_mempool

[jerome-jutteau]

we can init mempool global variable at pg_init time and use it instead of always check if mempool is created during pg_get_mempool.

• call pg_alloc_mempool() at pg_init()
• rename extern struct rte_mempool *mp to pg_mempool
• adapt every calls of pg_get_mempool and replace them with pg_mempool