MAINTAINERS: Add header files to NETWORKING sections

This is part of an effort to assign a section in MAINTAINERS to header
files that relate to Networking. In this case the files with "net" or
"skbuff" in their name.

This patch adds a number of such files to the NETWORKING DRIVERS
and NETWORKING [GENERAL] sections.

Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>