loop: remove lo_refcount and avoid lo_mutex in ->open / ->release

loop: remove lo_refcount and avoid lo_mutex in ->open / ->release

lo_refcount counts how many openers a loop device has, but that count
is already provided by the block layer in the bd_openers field of the
whole-disk block_device.  Remove lo_refcount and allow opens to
succeed even on devices beeing deleted - now that ->free_disk is
implemented we can handle that race gracefull and all I/O on it will
just fail. Similarly there is a small race window now where
loop_control_remove does not synchronize the delete vs the remove
due do bd_openers not being under lo_mutex protection, but we can
handle that just as gracefully.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20220330052917.2566582-15-hch@lst.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>