zbd: Avoid duplicating the code for calculating the number of sectors with data

zbd: Avoid duplicating the code for calculating the number of sectors with data

Introduce a new function process_swd() to avoid that almost identical code
occurs in check_swd() and zbd_file_reset(). Change the #if 0 / #endif pair
in check_swd() into if (false) to make sure that the compiler verifies the
syntactical correctness of that code.

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>