bpf: grab rcu read lock for bpf_percpu_hash_update

bpf_percpu_hash_update() expects rcu lock to be held and warns if it's not,
which pointed out a missing rcu read lock.

Fixes: 15a07b33 ("bpf: add lookup/update support for per-cpu hash and array maps")
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>