NFS: Add Anna Schumaker as co-maintainer for the NFS client

Anna has essentially been performing the duties of co-maintainer for
the past several years. In recognition of those efforts, I'd like to
add her to the maintainers file.

Cc: Anna Schumaker <anna.schumaker@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>