zbd: rename the accounting 'sectors with data' to 'valid data bytes'

zbd: rename the accounting 'sectors with data' to 'valid data bytes'

The 'sectors with data' accounting was designed to have 'sector' as its
unit. Then related variables have the word 'sector' in their names. Also
related code comments use the words 'sector' or 'logical blocks'.
However, actually it was implemented to have 'byte' as unit. Rename
related variables and comments to indicate the byte unit. Also replace
the abbreviation swd to vdb.

Fixes: a7c2b6fc2959 ("Add support for resetting zones periodically")
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>