Use improved credits estimates for quota operations.  Also reserve a space
for a quota operation in a transaction only if filesystem was mounted with
some quota options.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>