s390/bpf: Centralize frame offset calculations

The calculation of the distance from %r15 to the caller-allocated
portion of the stack frame is copy-pasted into multiple places in the
JIT code.

Move it to bpf_jit_prog() and save the result into bpf_jit::frame_off,
so that the other parts of the JIT can use it.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250624121501.50536-2-iii@linux.ibm.com


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