btrfs: remove the cross file system checks from remap

The sb check is already done in do_clone_file_range, and the mnt check
(which will hopefully go away in a subsequent patch) is done in
ioctl_file_clone().  Remove the check in our code and put an ASSERT() to
make sure it doesn't change underneath us.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>