bpf: Add verifier support for bpf_timer argument in kfuncs

bpf: Add verifier support for bpf_timer argument in kfuncs

Extend the verifier to recognize struct bpf_timer as a valid kfunc
argument type. Previously, bpf_timer was only supported in BPF helpers.

This prepares for adding timer-related kfuncs in subsequent patches.

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20260201025403.66625-3-alexei.starovoitov@gmail.com