LU-12988 ldiskfs: skip non-loaded groups at cr=0/1

cr=0 is supposed to be an optimization to save CPU cycles,
but if buddy data (in memory) is not initialized then all
this makes no sense as we have to do sync IO taking a lot
of cycles.  also, at cr=0 mballoc doesn't store any avaibale
chunk. cr=1 also skips groups using heruistic based on avg.
fragment size.
it's more useful to skip such groups and switch to cr=2 where
groups will be scanned for available chunks.

using sparse image and dm-slow virtual device of 120TB was
simulated. then the image was formatted as OST and filled
using debugfs to mark ~85% of available space as busy.
mount as OST w/o the patch couldn't complete in half an hour
(according to vmstat it would take ~10-11 hours). with the
patch applied mount took ~20 seconds.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I88c8c1b01b386af0fa438bfeb97acb6110bd00ec
Reviewed-on: https://review.whamcloud.com/36891
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Artem Blagodarenko <c17828@cray.com>