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: Peng Tao <bergwolf@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>