tracing, x86: fix constraint for parent variable

The constraint used for retrieving and restoring the parent function
pointer is incorrect. The parent variable is a pointer, and the
address of the pointer is modified by the asm statement and not
the pointer itself. It is incorrect to pass it in as an output
constraint since the asm will never update the pointer.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>