nfsd: fix NULL dereference in setattr()

The original code would oops if this were called from nfsd4_setattr()
because "filpp" is NULL.

(Note this case is currently impossible, as long as we only give out
read delegations.)

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>