[NETFILTER]: nf_conntrack: Fix module refcount dropping too far

If nf_ct_l3proto_find_get() fails to get the refcount of
nf_ct_l3proto_generic, nf_ct_l3proto_put() will drop the refcount
too far.

This gets rid of '.me = THIS_MODULE' of nf_ct_l3proto_generic so that
nf_ct_l3proto_find_get() doesn't try to get refcount of it.
It's OK because its symbol is usable until nf_conntrack.ko is unloaded.

This also kills unnecessary NULL pointer check as well.
__nf_ct_proto_find() allways returns non-NULL pointer.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>