os/bluestore: ignore 0x2000~2000 extent oddity from luminous upgrade

os/bluestore: ignore 0x2000~2000 extent oddity from luminous upgrade

Luminous does a block_size granularity freelist, and assumes that
0~ROUND_UP_TO(SUPER_RESERVED,block_size) is used.  Current master uses
min_alloc_size granularity and changes that assumption to
0~ROUND_UP_TO(SUPER_RESERVED,min_alloc_size).  That means if master
fsck's a luminous-created bluestore, it will think 0x2000~2000 is used
(current baked-in min_alloc_size-based assumption) but the old freelist
says it is free (old mkfs assumption).  The disparity is harmless since
the extent is below min_alloc_size, so ignore it.

Fixes: http://tracker.ceph.com/issues/21408
Signed-off-by: Sage Weil <sage@redhat.com>