gfs2: Use qd_sbd more consequently

Before this patch many of the functions in quota.c got their superblock
pointer, sdp, from the quota_data's glock pointer. That's silly because
the qd already has its own pointer to the superblock (qd_sbd).

This patch changes references to use that instead, eliminating a level
of indirection.

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