GFS2: Allow gfs2 to update quota usage values through the quotactl interface

With this patch the gfs2_set_dqblk() function will be able to update the
quota usage block count (FS_DQ_BCOUNT) in addition to the already supported
FS_DQ_BHARD (limit) and FS_DQ_BSOFT (warn) fields of the dquot structure.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>