gluster: Switch to .bdrv_co_block_status()

We are gradually moving away from sector-based interfaces, towards
byte-based.  Update the gluster driver accordingly.

In want_zero mode, we continue to report fine-grained hole
information (the caller wants as much mapping detail as possible);
but when not in that mode, the caller prefers larger *pnum and
merely cares about what offsets are allocated at this layer, rather
than where the holes live.  Since holes still read as zeroes at
this layer (rather than deferring to a backing layer), we can take
the shortcut of skipping find_allocation(), and merely state that
all bytes are allocated.

We can also drop redundant bounds checks that are already
guaranteed by the block layer.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>