uprobes/x86: Emulate nop's using ops->emulate()

Finally we can kill the ugly (and very limited) code in __skip_sstep().
Just change branch_setup_xol_ops() to treat "nop" as jmp to the next insn.

Thanks to lib/insn.c, it is clever enough. OPCODE1() == 0x90 includes
"(rep;)+ nop;" at least, and (afaics) much more.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jim Keniston <jkenisto@us.ibm.com>