selftests/bpf: Rename bpf_iter_task_vma.c to bpf_iter_task_vmas.c

Further patches in this series will add a struct bpf_iter_task_vma,
which will result in a name collision with the selftest prog renamed in
this patch. Rename the selftest to avoid the collision.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231013204426.1074286-3-davemarchevsky@fb.com