bpf: Fix a data-race around bpf_jit_limit.

While reading bpf_jit_limit, it can be changed concurrently via sysctl,
WRITE_ONCE() in __do_proc_doulongvec_minmax(). The size of bpf_jit_limit
is long, so we need to add a paired READ_ONCE() to avoid load-tearing.

Fixes: ede95a63 ("bpf: add bpf_jit_limit knob to restrict unpriv allocations")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220823215804.2177-1-kuniyu@amazon.com