zbd: Restore zbd_check_swd()

zbd: Restore zbd_check_swd()

A recent patch removed the check_swd() function. Restore it since this function
is useful when debugging fio. Rename the function into zbd_check_swd(). Insert
the calls to these function where no locks are held to avoid triggering a
deadlock. See also commit d60be7d51cbb ("zbd: Remove unused function and
variable").

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>