vmpressure: change vmpressure::sr_lock to spinlock

vmpressure: change vmpressure::sr_lock to spinlock

There is nothing that can sleep inside critical sections protected by
this lock and those sections are really small so there doesn't make much
sense to use mutex for them.  Change the log to a spinlock

Signed-off-by: Michal Hocko <[email protected]>
Reported-by: Tejun Heo <[email protected]>
Cc: Anton Vorontsov <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Li Zefan <[email protected]>
Reviewed-by: Tejun Heo <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>