PM: hibernate: Rename function parameter from snapshot_test to exclusive

Several functions reply on snapshot_test to decide whether to
open the resume device exclusively. However there is no strict
connection between the snapshot_test and the open mode. Rename
the 'snapshot_test' input parameter to 'exclusive' to better reflect
the use case.

No functional change is expected.

Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>