selftests/bpf: Test all valid alloc sizes for bpf mem allocator

Add a test to test all possible and valid allocation size for bpf
memory allocator. For each possible allocation size, the test uses
the following two steps to test the alloc and free path:

1) allocate N (N > high_watermark) objects to trigger the refill
   executed in irq_work.
2) free N objects to trigger the freeing executed in irq_work.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230908133923.2675053-5-houtao@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>