nfsd4: remove pointless strdup_if_nonnull

kstrdup() already checks for NULL.

(Brought to our attention by Jason Yann noticing (from sparse output)
that it should have been declared static.)

Signed-off-by: NeilBrown <neilb@suse.com>
Reported-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>