MAINTAINERS: Fixes to Networking Buffers section

Add a missing c-file (buf_simple.c) and use a dedicated label for
Networking Buffers. Also add explicit excludes of the same files to the
Networking section. Without the dedicated label it seems that the wrong
maintainer gets set as assignee for pull requests.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>