net: shell: Add cmd to see net_buf allocations

This commit adds "net allocs" command that can be used to
track net_buf allocations that are done by net_nbuf_get*()
and net_nbuf_unref().

Change-Id: If3dc2ecf5552f8008138ee9733458a19f9764c13
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>