NFSD: make cache_detail structures const

Make these const as they are only getting passed to the function
cache_create_net having the argument as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>