powerpc/pseries/hibernation: pass stream id via function arguments

There is no need for the stream id to be a file-global variable; pass
it from hibernate_store() to pseries_suspend_begin() for the
H_VASI_STATE call.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201207215200.1785968-19-nathanl@linux.ibm.com