bpf: extract same_callsites() as utility function

Extract same_callsites() from clean_live_states() as a utility function.
This function would be used by the next patch in the set.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20231024000917.12153-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>