GFS2: BUG in gfs2_adjust_quota

GFS2: BUG in gfs2_adjust_quota

commit 8b4216018bdbfbb1b76150d202b15ee68c38e991 upstream.

HighMem pages on i686 do not get mapped to the buffer_heads and this was
causing a NULL pointer dereference when we were trying to memset page buffers
to zero.
We now use zero_user() that kmaps the page and directly manipulates page data.
This patch also fixes a boundary condition that was incorrect.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>