uprobes/x86: Change __skip_sstep() to actually skip the whole insn

__skip_sstep() doesn't update regs->ip. Currently this is correct
but only "by accident" and it doesn't skip the whole insn. Change
it to advance ->ip by the length of the detected 0x66*0x90 sequence.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>