contigmem: fix buffer overrun on unload

contigmem: fix buffer overrun on unload

The maximum mount contiguous memory regions for FreeBSD is limited by
RTE_CONTIGMEM_MAX_NUM_BUFS, a pointer to each region is stored in
static void * contigmem_buffers[RTE_CONTIGMEM_MAX_NUM_BUFS]

A user can specify a greater amount via hw.contigmem.num_buffers,
while the allocation logic will prevent this allocation from occuring the logic
in contigmem_unload() will attempt to free hw.contigmem.num_buffers and an
overrun occurs.

This patch limits the freeing to a maximum of RTE_CONTIGMEM_MAX_NUM_BUFS.

Signed-off-by: Alan Carew <alan.carew@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>