efi: Don't use spinlocks for efi vars

efi: Don't use spinlocks for efi vars

All efivars operations are protected by a spinlock which prevents
interruptions and preemption. This is too restricted, we just need a
lock preventing concurrency.
The idea is to use a semaphore of count 1 and to have two ways of
locking, depending on the context:
- In interrupt context, we call down_trylock(), if it fails we return
  an error
- In normal context, we call down_interruptible()

We don't use a mutex here because the mutex_trylock() function must not
be called from interrupt context, whereas the down_trylock() can.

Signed-off-by: Sylvain Chouleur <[email protected]>
Signed-off-by: Ard Biesheuvel <[email protected]>
Cc: Leif Lindholm <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Sylvain Chouleur <[email protected]>
Signed-off-by: Matt Fleming <[email protected]>