context_tracking: Restore previous state in schedule_user

context_tracking: Restore previous state in schedule_user

It appears that some SCHEDULE_USER (asm for schedule_user) callers
in arch/x86/kernel/entry_64.S are called from RCU kernel context,
and schedule_user will return in RCU user context.  This causes RCU
warnings and possible failures.

This is intended to be a minimal fix suitable for 3.18.

Reported-and-tested-by: Dave Jones <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Frédéric Weisbecker <[email protected]>
Acked-by: Paul E. McKenney <[email protected]>
Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>