PM: hibernate: Turn snapshot_test into global variable

[ Upstream commit 08169a16 ]

There is need to check snapshot_test and open block device
in different mode, so as to avoid the race condition.

No functional changes intended.

Suggested-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Stable-dep-of: 5904de0d ("PM: hibernate: Do not get block device exclusively in test_resume mode")
Signed-off-by: Sasha Levin <sashal@kernel.org>