s390/bpf: Use bcr 0,%0 as tail call nop filler

Currently used 0x0000 filler confuses bfd disassembler, making bpftool
prog dump xlated output nearly useless. Fix by using a real instruction.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200602174555.2501389-1-iii@linux.ibm.com