nfsd: make nfsd4_encode_fattr static

sparse says:

      CHECK   fs/nfsd/nfs4xdr.c
    fs/nfsd/nfs4xdr.c:2043:1: warning: symbol 'nfsd4_encode_fattr' was not declared. Should it be static?

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>