net/9p/client.c: fix potential refcnt problem of trans module

net/9p/client.c: fix potential refcnt problem of trans module

When specifying trans_mod multiple times in a mount, it will cause an
inaccurate refcount of the trans module.  Also, in the error case of
option parsing, we should put the trans module if we have already got
it.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Chengguang Xu <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: David Miller <[email protected]>
Cc: Eric Van Hensbergen <[email protected]>
Cc: Ron Minnich <[email protected]>
Cc: Latchesar Ionkov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>