[PATCH] uml: add locking to network transport registration

The registration of host network transports needed some locking.  The
transport list itself is locked, but calls to the registration routines are
not.  This is compensated for by checking that a transport structure is not
yet on any list.

I also took the opportunity to const all fields in the transport structure
except the list, which obviously can be modified.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Cc: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>