Use direct 64-bit conversion when computing shift

In mark_swap() and reset_signature() in suspend.c and in
open_resume_dev() and reset_signature() in resume.c the value of
shift is computed as a 32-bit value (on 32-bit systems), so use
direct type casting to make it 64-bit.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Reported-by: Jakob Lell <jakob@cs.tu-berlin.de>