ext4: fix build warning when EXT4FS_DEBUG is on

When compiling with EXT4FS_DEBUG on, gcc will complain with following warnings:

linux-2.6/fs/ext4/ialloc.c: In function ‘ext4_count_free_inodes’:
linux-2.6/fs/ext4/ialloc.c:1192: warning: format ‘%lu’ expects type
‘long unsigned int’, but argument 2 has type ‘ext4_group_t’

So add a type cast to suppress it. 
Signed-off-by: NPeng Tao <bergwolf@gmail.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/ialloc.c

@@ -1189,7 +1189,7 @@ unsigned long ext4_count_free_inodes(struct super_block *sb)
 
 		x = ext4_count_free(bitmap_bh, EXT4_INODES_PER_GROUP(sb) / 8);
 		printk(KERN_DEBUG "group %lu: stored = %d, counted = %lu\n",
-			i, ext4_free_inodes_count(sb, gdp), x);
+			(unsigned long) i, ext4_free_inodes_count(sb, gdp), x);
 		bitmap_count += x;
 	}
 	brelse(bitmap_bh);