s390/bpf: Store backchain even for leaf progs

[ Upstream commit 5f55f216 ]

Currently a crash in a leaf prog (caused by a bug) produces the
following call trace:

     [<000003ff600ebf00>] bpf_prog_6df0139e1fbf2789_fentry+0x20/0x78
     [<0000000000000000>] 0x0

This is because leaf progs do not store backchain. Fix by making all
progs do it. This is what GCC and Clang-generated code does as well.
Now the call trace looks like this:

     [<000003ff600eb0f2>] bpf_prog_6df0139e1fbf2789_fentry+0x2a/0x80
     [<000003ff600ed096>] bpf_trampoline_201863462940+0x96/0xf4
     [<000003ff600e3a40>] bpf_prog_05f379658fdd72f2_classifier_0+0x58/0xc0
     [<000003ffe0aef070>] bpf_test_run+0x210/0x390
     [<000003ffe0af0dc2>] bpf_prog_test_run_skb+0x25a/0x668
     [<000003ffe038a90e>] __sys_bpf+0xa46/0xdb0
     [<000003ffe038ad0c>] __s390x_sys_bpf+0x44/0x50
     [<000003ffe0defea8>] __do_syscall+0x150/0x280
     [<000003ffe0e01d5c>] system_call+0x74/0x98

Fixes: 05462310 ("s390/bpf: Add s390x eBPF JIT compiler backend")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250512122717.54878-1-iii@linux.ibm.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>