kni: allocate no more mbuf than empty slots in queue

kni: allocate no more mbuf than empty slots in queue

In kni_allocate_mbufs(), we attempt to add max_burst (32) count of mbuf
always into alloc_q, which is excessively leading too many rte_pktmbuf_
free() when alloc_q is contending at high packet rate (for eg 10Gig data).
In a situation when alloc_q fifo can only accommodate very few (or zero)
mbuf, create only what needed and add in fifo.

With this patch, we could stop random network stall in KNI at higher packet
rate (eg 1G or 10G data between vEth0 and PMD) sufficiently exhausting
alloc_q on above condition. I tested i40e PMD for this purpose in ppc64le.

Signed-off-by: Gowrishankar Muthukrishnan <gowrishankar.m@linux.vnet.ibm.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>