Storing caller's ip in trampoline's stack. Trampoline programs
can reach the IP in (ctx - 8) address, so there's no change in
program's arguments interface.

The IP address is takes from [fp + 8], which is return address
from the initial 'call fentry' call to trampoline.

This IP address will be returned via bpf_get_func_ip helper
helper, which is added in following patches.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210714094400.396467-2-jolsa@kernel.org