[PATCH] powerpc/ftrace: restore r2 to caller's stack on livepatch sibling call

[Ryan Sullivan]

Hi Michael,

The r2 value is stored to the livepatch stack prior to entering into 
the livepatched code, so accessing it will gurantee the previous value
is restored.

Also, yes, this bug is caused by tooling that "scoops out" pre-compiled
code and places it into the livepatch handler (e.g. kpatch). However, 
since the large majority of customers interact with the livepatch 
subsystem through tooling, and this fix would not pose any serious risk
to either usability or security (other than those already present in 
livepatching), plus it would solve a large problem for these tools with
a simple fix, I feel as though this would be a useful update to 
livepatch.

Thanks,

Ryan