zonefs: Rename super block information fields

The s_open_zones field of struct zonefs_sb_info is used to count the
number of files that are open for writing and may not necessarilly
correspond to the number of open zones on the device. For instance, an
application may open for writing a sequential zone file, fully write it
and keep the file open. In such case, the zone of the file is not open
anymore (it is in the full state).

Avoid confusion about this counter meaning by renaming it to
s_wro_seq_files. To keep things consistent, the field s_max_open_zones
is renamed to s_max_wro_seq_files.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>