zonefs: Reduce struct zonefs_inode_info size

Instead of using the i_ztype field in struct zonefs_inode_info to
indicate the zone type of an inode, introduce the new inode flag
ZONEFS_ZONE_CNV to be set in the i_flags field of struct
zonefs_inode_info to identify conventional zones. If this flag is not
set, the zone of an inode is considered to be a sequential zone.

The helpers zonefs_zone_is_cnv(), zonefs_zone_is_seq(),
zonefs_inode_is_cnv() and zonefs_inode_is_seq() are introduced to
simplify testing the zone type of a struct zonefs_inode_info and of a
struct inode.

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