bpf: Move sleepable verification code to btf_id_allow_sleepable

bpf: Move sleepable verification code to btf_id_allow_sleepable

Move sleepable verification code to btf_id_allow_sleepable function.
It will be used in following changes.

Adding code to retrieve type's name instead of passing it from
bpf_check_attach_target function, because this function will be
called from another place in following changes and it's easier
to retrieve the name directly in here.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260606123955.345967-12-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>