selftests/bpf: Add three test cases for bits_iter

Add more test cases for bits iterator:

(1) huge word test
Verify the multiplication overflow of nr_bits in bits_iter. Without
the overflow check, when nr_words is 67108865, nr_bits becomes 64,
causing bpf_probe_read_kernel_common() to corrupt the stack.
(2) max word test
Verify correct handling of maximum nr_words value (511).
(3) bad word test
Verify early termination of bits iteration when bits iterator
initialization fails.

Also rename bits_nomem to bits_too_big to better reflect its purpose.

Signed-off-by: Hou Tao <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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