bpf: selftests: Fix bpf_iter_task_vma_new() prototype

bpf_iter_task_vma_new() is defined as taking a u64 as its 3rd argument.
u64 is a unsigned long long. bpf_experimental.h was defining the
prototype as unsigned long.

Fix by using __u64.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/fab4509bfee914f539166a91c3ff41e949f3df30.1718207789.git.dxu@dxuuu.xyz


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