[NET]: Fix uninitialised variable in ip_frag_reasm()

Fix uninitialised variable in ip_frag_reasm().  err should be set to
-ENOMEM if the initial call of skb_clone() fails.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>