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: 15a07b338 ("bpf: add lookup/update support for per-cpu hash and array maps")
Signed-off-by: Sasha Levin <redacted>
---
 kernel/bpf/syscall.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index c95a753..94324b8 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -348,7 +348,9 @@ static int map_update_elem(union bpf_attr *attr)
 		goto free_value;
 
 	if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH) {
+		rcu_read_lock();
 		err = bpf_percpu_hash_update(map, key, value, attr->flags);
+		rcu_read_unlock();