[4.14] powerpc: Always initialize input array when calling epapr_hypercall()

Kernel 4.14 fails to build with GCC 8 on powerpc64, due to 'in' being uninitialised in epapr_hypercall*.

This is fixed in commit 186b8f1587c79c2fa04bfa392fdf08 upstream, and this commit applies cleanly to the 4.14 tree. This commit is already on the 4.19 branch.

Best, --arw