crimson/zns: Add zone-capacity support.

crimson/zns: Add zone-capacity support.

ZNS SSDs have an attribute called zone_capacity which can be less than or
equal to zone_size. zone_capacity represents the actual writable media in
a zone. When zone_capacity is less than zone_size, writing to offsets
beyond zone_capacity will cause write errors.
Set the segment size as equal to zone_capacity, so that segment managers
writes only upto capacity of the zone/segment.
Update device size to actual available bytes so that the gc can kick in
at appropriate thresholds.

Signed-off-by: Aravind Ramesh <aravind.ramesh@wdc.com>