bpf: Fix out of bounds memory access in bpf_sk_storage

bpf_sk_storage maps use multiple spin locks to reduce contention.
The number of locks to use is determined by the number of possible CPUs.
With only 1 possible CPU, bucket_log == 0, and 2^0 = 1 locks are used.

When updating elements, the correct lock is determined with hash_ptr().
Calling hash_ptr() with 0 bits is undefined behavior, as it does:

x >> (64 - bits)

Using the value results in an out of bounds memory access.
In my case, this manifested itself as a page fault when raw_spin_lock_bh()
is called later, when running the self tests:

./tools/testing/selftests/bpf/test_verifier 773 775
[   16.366342] BUG: unable to handle page fault for address: ffff8fe7a66f93f8

Force the minimum number of locks to two.

Signed-off-by: Arthur Fabre <afabre@cloudflare.com>
Fixes: 6ac99e8f

 ("bpf: Introduce bpf sk local storage")
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>