s390/bpf: use 32-bit index for tail calls

"p runtime/jit: pass > 32bit index to tail_call" fails when
bpf_jit_enable=1, because the tail call is not executed.

This in turn is because the generated code assumes index is 64-bit,
while it must be 32-bit, and as a result prog array bounds check fails,
while it should pass. Even if bounds check would have passed, the code
that follows uses 64-bit index to compute prog array offset.

Fix by using clrj instead of clgrj for comparing index with array size,
and also by using llgfr for truncating index to 32 bits before using it
to compute prog array offset.

Fixes: 6651ee07 ("s390/bpf: implement bpf_tail_call() helper")
Reported-by: Yauheni Kaliuta <yauheni.kaliuta@redhat.com>
Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>