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 <sylvain.chouleur@intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Sylvain Chouleur <sylvain.chouleur@gmail.com>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>

fs/efivarfs/inode.c

@@ -105,7 +105,10 @@ static int efivarfs_create(struct inode *dir, struct dentry *dentry,
 
 	inode->i_private = var;
 
-	efivar_entry_add(var, &efivarfs_list);
+	err = efivar_entry_add(var, &efivarfs_list);
+	if (err)
+		goto out;
+
 	d_instantiate(dentry, inode);
 	dget(dentry);
 out: