ext3: Avoid loading bitmaps for full groups during block allocation

There is no point in loading bitmap for groups which are completely full.
This causes noticeable performance problems (and memory pressure) on small
systems with large full filesystem
(http://marc.info/?l=linux-ext4&m=126843108314310&w=2

).

Jan Kara: Added a comment and changed check to use cpu-endian value.

Signed-off-by: "Frans van de Wiel" <fvdw@fvdw.eu>
Signed-off-by: Jan Kara <jack@suse.cz>