[NETFILTER]: nf_conntrack: simplify l4 protocol array allocation

The retrying after an allocation failure is not necessary anymore
since we're holding the mutex the entire time, for the same
reason the double allocation race can't happen anymore.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>