ext4: After completely filled group, scan next group from the beginning

The last free block of a block group may be in its middle. After it has
been allocated, the next block group should be scanned from its beginning.

The following command triggers the bad behaviour (on a blocksize 1024 fs):

./sandbox/u-boot -c 'i=0; host bind 0 ./disk.raw ;
	while test $i -lt 260 ; do echo $i; setexpr i $i + 1;
		ext4write host 0:2 0 /X${i} 0x1450; done ;
	ext4write host 0:2 0 /X240 0x2000 ; '

When 'X240' is extended from 5200 byte to 8192 byte, the new blocks should
start from the first free block (8811), but it uses the blocks 8098-8103
and 16296-16297 -- 8103 + 1 + 8192 = 16296. This can be shown with
debugfs, commands 'ffb' and 'stat X240'.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>