gfs2: quota need_sync cleanup

Rename variable 'value' to 'change' as it stores a change in value.

Add new 'value' and 'limit' variables for the current value and limit.

Only fetch the tuning parameters when we need them.

Get rid of unnecessary nesting.

No change in functionality.

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