9p: Remove INET dependency

9pfs can run over assorted transports, so it doesn't have an INET
dependency.  Drop it and remove the includes of linux/inet.h.

NET_9P_FD/trans_fd.o builds without INET or UNIX and is usable over
plain file descriptors.  However, tcp and unix functionality is still
built and would generate runtime failures if used.  Add imply INET and
UNIX to NET_9P_FD, so functionality is enabled by default but can still
be explicitly disabled.

This allows configuring 9pfs over Xen with INET and UNIX disabled.

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>