nfsd: fix v4 reply caching

commit 57d276d71aef7d8305ff002a070cb98deb2edced upstream.

Very embarassing: 1091006c5eb15cba56785bd5b498a8d0b9546903 "nfsd: turn
on reply cache for NFSv4" missed a line, effectively leaving the reply
cache off in the v4 case.  I thought I'd tested that, but I guess not.

This time, wrote a pynfs test to confirm it works.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>