powerpc/kvm: Fix unannotated intra-function call warning

objtool throws the following warning:
  arch/powerpc/kvm/booke.o: warning: objtool: kvmppc_fill_pt_regs+0x30:
  unannotated intra-function call

Fix the warning by setting the value of 'nip' using the _THIS_IP_ macro,
without using an assembly bl/mflr sequence to save the instruction
pointer.

Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Sathvika Vasireddy <sv@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20230128124158.1066251-1-sv@linux.ibm.com