nbd/server: Nicer spelling of max BLOCK_STATUS reply length

nbd/server: Nicer spelling of max BLOCK_STATUS reply length

Commit 3d068aff (3.0) introduced NBD_MAX_BITMAP_EXTENTS as a limit on
how large we would allow a reply to NBD_CMD_BLOCK_STATUS to grow when
it is visiting a qemu:dirty-bitmap: context.  Later, commit fb7afc79
(3.1) reused the constant to limit base:allocation context replies,
although the name is now less appropriate in that situation.

Rename things, and improve the macro to use units.h for better
legibility. Then reformat the comment to comply with checkpatch rules
added in the meantime. No semantic change.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190510151735.29687-1-eblake@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>