riscv, bpf: Fix broken BPF tail calls

riscv, bpf: Fix broken BPF tail calls

commit f1003b787c00fbaa4b11619c6b23a885bfce8f07 upstream.

The BPF JIT incorrectly clobbered the a0 register, and did not flag
usage of s5 register when BPF stack was being used.

Fixes: 2353ecc6f91f ("bpf, riscv: add BPF JIT for RV64G")
Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191216091343.23260-2-bjorn.topel@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>