9p: return NULL when trans not found

v9fs_match_trans function returns arbitrary transport module instead of NULL
when the requested transport is not registered. This patch modifies the
function to return NULL in that case.

Signed-off-by: Latchesar Ionkov <lucho@ionkov.net>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>

diff --cc net/9p/mod.c
Simple merge