os/bluestore: record gifts < expected size in superblock

os/bluestore: record gifts < expected size in superblock

Returning 0 from _balance_bluefs_freespace() skips recording
allocations in the superblock, so we fail the consistency check on
startup. The elseif branch handles this case already, so just remove
it from the first branch. This is luminous/mimic specific, since
bluefs extents are not recorded in the superblock in later releases.

Signed-off-by: Josh Durgin <jdurgin@redhat.com>