[CORE] net: Simple slab based allocater for mbuf external storage

This patch implements simple slab based allocater for mbuf external storage.

Basically, we create 4 mempools for buffer sizes 256, 512, 1024, and 2048 bytes. Each mempool is an allocater of fixed size buffers.

The m_ext_get() functions selects one of the 4 mempools based on size of external storage required.

Signed-off-by: Anup Patel <anup@brainfault.org>