LU-2093 lod: fall back to RR allocation when QoS fails

LU-2093 lod: fall back to RR allocation when QoS fails

lod_alloc_qos() checks is there enough OSPs to satisfy the request
checking OSP state with dt_statfs(), then it tries to reserve
objects on some of them. during the reservation the state of OSP
can change (due to broken connection, for example), then QoS code
might found less ready OSPs than required. this is a valid situation
and LOD should fallback to RR allocation.

sanity/116a added to verify this: dt_statfs() are still reporting
OSPs are good, but no actual object can be created on OSP with
index 1.

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Iae6916f998070960eb47c71f2bc1e48adb2ac080
Reviewed-on: http://review.whamcloud.com/4241
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Tested-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>