gfs2: pass sdp in to gfs2_write_disk_quota

Like the previous patch, we now pass the superblock pointer to function
gfs2_write_disk_quota. This makes the code more understandable, since it
only operates on the quota inode.

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