udp: get rid of SLAB_DESTROY_BY_RCU allocations

After commit ca065d0c

 ("udp: no longer use SLAB_DESTROY_BY_RCU")
we do not need this special allocation mode anymore, even if it is
harmless.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>