LU-748 test: shorten the runtime of sanity subtest_220

LU-748 test: shorten the runtime of sanity subtest_220

in sanity.sh, test_220 tries to exhaust all of the inodes on the OSTs
in order to verify that when it returns -ENOSPC to inode precreate
request, but there is still free blocks, then the MDS continues to use
these precreated inodes on the OSTs.

Signed-off-by: Hongchao Zhang <hongchao.zhang@whamcloud.com>
Change-Id: Icaad07311125f362f0efb26da76534c7dca27b6a
Reviewed-on: http://review.whamcloud.com/1676
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>