powerpc/pseries: hvcall stack frame overhead

This call may use the min size stack frame. The scratch space used is
in the caller's parameter area frame, not this function's frame.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221127124942.1665522-6-npiggin@gmail.com