LU-5246 tests: create OST objects on correct MDT

LU-5246 tests: create OST objects on correct MDT

The sanity.sh test_220 is checking that precreated objects on the MDT
can be used even when the OST has subsequently run out of inodes.
The test was not correctly working if the directory was striped over
multiple MDTs since the number of precreated objects on other MDTs
may be fewer than the number of precreated objects $SINGLEMDS and
run out of objects.

Change the test to only create files on $SINGLEMDS so to avoid this.

Also fix the return code of lod_alloc_specific() to be -ENOSPC if
the stripe count is 1, since -EFBIG doesn't make sense in that case.
Fix other code comments for lod_alloc_specific() and lod_alloc_qos().

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I788706c3b212b426aa15424d02913aedcdb98b2a
Reviewed-on: http://review.whamcloud.com/12937
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>