ext4: fix compile error while opening the macro DOUBLE_CHECK

the error is:
    fs/ext4/mballoc.c:475:43: error: 'struct ext4_group_info' has
no member named 'bb_bitmap'.
    so, the definition of macro DOUBLE_CHECK should before
'struct ext4_group_info', I fixed it, and I moved the macro
AGGRESSIVE_CHECK together, because I think they shoule be together.

Signed-off-by: Aihua Zhang <zhangaihua1@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>