blkzone: don't show wptr when zones are full

blkzone: don't show wptr when zones are full

blkzone report displays the write pointer when zones are full, however, the
writepointer value is not accurate for full zones as all zone LBAs have been
written. Instead, fix this so that we show "N/A" for the writepointer field
if the zone condition is full.

While we are at it, extend this behavior for convetional zones instead
of displaying 0 for the writepointer.

Signed-off-by: Wilfred Mallawa <wilfred.mallawa@wdc.com>