arm, bpf: Fix bugs with ALU64 {RSH, ARSH} BPF_K shift by 0

The current arm BPF JIT does not correctly compile RSH or ARSH when the
immediate shift amount is 0. This causes the "rsh64 by 0 imm" and "arsh64
by 0 imm" BPF selftests to hang the kernel by reaching an instruction
the verifier determines to be unreachable.

The root cause is in how immediate right shifts are encoded on arm.
For LSR and ASR (logical and arithmetic right shift), a bit-pattern
of 00000 in the immediate encodes a shift amount of 32. When the BPF
immediate is 0, the generated code shifts by 32 instead of the expected
behavior (a no-op).

This patch fixes the bugs by adding an additional check if the BPF
immediate is 0. After the change, the above mentioned BPF selftests pass.

Fixes: 39c13c20

 ("arm: eBPF JIT compiler")
Co-developed-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200408181229.10909-1-luke.r.nels@gmail.com