block: change wait on bd_claiming to use a var_waitqueue

block: change wait on bd_claiming to use a var_waitqueue

bd_prepare_to_claim() waits for a var to change, not for a bit to be
cleared. Change from bit_waitqueue() to __var_waitqueue() and
correspondingly use wake_up_var(). This will allow a future patch which
change the "bit" function to expect an "unsigned long *" instead of
"void *".

Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/20240826063659.15327-2-neilb@suse.de
Signed-off-by: Jens Axboe <axboe@kernel.dk>