nfsd: Add Jeff Layton as co-maintainer

Jeff has been doing a lot of development (including much of the
state-locking rewrite just as one example) plus lots of review and other
miscellaneous nfsd work, so let's acknowledge the status quo.

I'll continue to be the one to send regular pull requests but Jeff will
should be available to cover there occasionally too.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>