nfsd: break out comparator into separate function

Break out the function that compares the rqstp and checksum against a
reply cache entry. While we're at it, track the efficacy of the checksum
over the NFS data by tracking the cases where we would have incorrectly
matched a DRC entry if we had not tracked it or the length.

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