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>

fs/ext4/ext4_common.c

@@ -903,8 +903,8 @@ uint32_t ext4fs_get_new_blk_no(void)
 
 		goto fail;
 	} else {
-restart:
 		fs->curr_blkno++;
+restart:
 		/* get the blockbitmap index respective to blockno */
 		bg_idx = fs->curr_blkno / blk_per_grp;
 		if (fs->blksz == 1024) {
@@ -922,8 +922,9 @@ restart:
 
 		if (bgd[bg_idx].free_blocks == 0) {
 			debug("block group %u is full. Skipping\n", bg_idx);
-			fs->curr_blkno = fs->curr_blkno + blk_per_grp;
-			fs->curr_blkno--;
+			fs->curr_blkno = (bg_idx + 1) * blk_per_grp;
+			if (fs->blksz == 1024)
+				fs->curr_blkno += 1;
 			goto restart;
 		}
 
@@ -940,6 +941,7 @@ restart:
 				   bg_idx) != 0) {
 			debug("going for restart for the block no %ld %u\n",
 			      fs->curr_blkno, bg_idx);
+			fs->curr_blkno++;
 			goto restart;
 		}