GFS2: Userland expects quota limit/warn/usage in 512b blocks

Userland programs using the quotactl() syscall assume limit/warn/usage
block counts in 512b basic blocks which were instead being read/written
in fs blocksize in gfs2. With this patch, gfs2 correctly interacts with
the syscall using 512b blocks.

Signed-off-by: Abhi Das <adas@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>