host/mesh: Fix friend buf send end not called.
host/mesh: Fix friend buf send end not called.

As frnd->last will keep reference, so that net buffer
destructor function will not be call.

This is port of ecc7ca1b698bd2d3b7f043c945980b909cc3e4a7

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble