s390/bpf,jit: initialize A register if 1st insn is BPF_S_LDX_B_MSH

commit 6e0de817594c61f3b392a9245deeb09609ec707d upstream.

The A register needs to be initialized to zero in the prolog if the
first instruction of the BPF program is BPF_S_LDX_B_MSH to prevent
leaking the content of %r5 to user space.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>