s390/bpf: Support address space cast instruction

s390/bpf: Support address space cast instruction

The new address cast instruction translates arena offsets to userspace
addresses. NULL pointers must not be translated.

The common code sets up the mappings in such a way that it's enough to
replace the higher 32 bits to achieve the desired result. s390x has
just an instruction for this: INSERT IMMEDIATE.

Implement the sequence using 3 instruction: LOAD AND TEST, BRANCH
RELATIVE ON CONDITION and INSERT IMMEDIATE.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240701234304.14336-8-iii@linux.ibm.com