LU-3220 quota: warning on setting small limits

LU-3220 quota: warning on setting small limits

When user try to set quota limits smaller than the minimal qunit
size (1M bytes for block limit, 1K inodes for inode limit), we'd
warn user that limit is too small.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Change-Id: I26e2e682500628211e2f2bedf65e534f3b214422
Reviewed-on: http://review.whamcloud.com/6182
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>