gfs2: remove unneeded pg_oflow variable

Function gfs2_write_disk_quota checks if its write overflows onto
another page, and if so, does a second write. Before this patch it kept
two variables for this, but only one is needed. This patch simplifies
it by eliminating pg_oflow.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>