fnctl: f_modown should call write_lock_irqsave/restore

Commit 703625118069f9f8960d356676662d3db5a9d116 exposed that f_modown()
should call write_lock_irqsave instead of just write_lock_irq so that
because a caller could have a spinlock held and it would not be good to
renable interrupts.

Cc: Eric W. Biederman <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Alan Cox <[e-mail]>
Cc: Tavis Ormandy <[e-mail]>
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/fcntl.c

···
 static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
                      int force)
 {
-	write_lock_irq(&filp->f_owner.lock);
+	unsigned long flags;
+
+	write_lock_irqsave(&filp->f_owner.lock, flags);
 	if (force || !filp->f_owner.pid) {
 		put_pid(filp->f_owner.pid);
 		filp->f_owner.pid = get_pid(pid);
···
 			filp->f_owner.euid = cred->euid;
 		}
 	}
-	write_unlock_irq(&filp->f_owner.lock);
+	write_unlock_irqrestore(&filp->f_owner.lock, flags);
 }
 
 int __f_setown(struct file *filp, struct pid *pid, enum pid_type type,