loop: add recursion validation to LOOP_CHANGE_FD

Refactor the validation code used in LOOP_SET_FD so it is also used in
LOOP_CHANGE_FD.  Otherwise it is possible to construct a set of loop
devices that all refer to each other.  This can lead to a infinite
loop in starting with "while (is_loop_device(f)) .." in loop_set_fd().

Fix this by refactoring out the validation code and using it for
LOOP_CHANGE_FD as well as LOOP_SET_FD.

Reported-by:  <syzbot+4349872271ece473a7c91190b68b4bac7c5dbc87@syzkaller.appspotmail.com>
Reported-by:  <syzbot+40bd32c4d9a3cc12a339@syzkaller.appspotmail.com>
Reported-by:  <syzbot+769c54e66f994b041be7@syzkaller.appspotmail.com>
Reported-by:  <syzbot+0a89a9ce473936c57065@syzkaller.appspotmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>