btrfs: fix use of uninitialized variable at rm device ioctl

Clang static analysis reports this problem
ioctl.c:3333:8: warning: 3rd function call argument is an
  uninitialized value
    ret = exclop_start_or_cancel_reloc(fs_info,

cancel is only set in one branch of an if-check and is always used.  So
initialize to false.

Fixes: 1a15eb7 ("btrfs: use btrfs_get_dev_args_from_path in dev removal ioctls")
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ioctl.c

@@ -3353,7 +3353,7 @@ static long btrfs_ioctl_rm_dev(struct file *file, void __user *arg)
 	struct block_device *bdev = NULL;
 	fmode_t mode;
 	int ret;
-	bool cancel;
+	bool cancel = false;
 
 	if (!capable(CAP_SYS_ADMIN))
 		return -EPERM;