ext4: move ext4_mb_init_group() function earlier in the mballoc.c

This moves the function around so that it can be called from
ext4_mb_load_buddy().

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>