net/9p: clarify trans_fd parse_opt failure handling

This parse_opts will set invalid opts.rfd/wfd in case of failure which
we already check, but it is not clear for readers that parse_opts error
are handled in p9_fd_create: clarify this by explicitely checking the
return value.

Link: https://lkml.kernel.org/r/20220921210921.1654735-1-floridsleeves@gmail.com
Signed-off-by: Li Zhong <floridsleeves@gmail.com>
[Dominique: reworded commit message to clarify this is NOOP]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>