Revert "loop: Get rid of loop_index_mutex"

This reverts commit 6a8f1d8d which is
commit 0a42e99b58a208839626465af194cfe640ef9493 upstream.

It does not work properly in the 4.9.y tree and causes more problems
than it fixes, so revert it.

Reported-by: Thomas Lindroth <thomas.lindroth@gmail.com>
Reported-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>