lwipopts: increase MEMP_NUM_PBUF for send performance

MEMP_NUM_PBUF is the number of PBUF_REF-type pbufs, which points
to the data somewhere else instead of containing it.

For send traffic, no new regular pbuf is allocated from PBUF_POOL,
rather PBUF_REF-type pbuf is used to point to the application data.

Change-Id: Ia1d7e4c9705df4c44fef7b912e1a3f0f2abd2b66
1 file changed