bpf: Introduce sysctl kernel.bpf_force_dyn_alloc.

Introduce sysctl kernel.bpf_force_dyn_alloc to force dynamic allocation in bpf
hash map. All selftests/bpf should pass with bpf_force_dyn_alloc 0 or 1 and all
bpf programs (both sleepable and not) should not see any functional difference.
The sysctl's observable behavior should only be improved memory usage.

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>