ext4: optimize the implementation of ext4_mb_good_group()

ext4: optimize the implementation of ext4_mb_good_group()

It might be better to adjust the code in two places:
1. Determine whether grp is currupt or not should be placed first.
2. (cr<=2 && free <ac->ac_g_ex.fe_len)should may belong to the crx
   strategy, and it may be more appropriate to put it in the
   subsequent switch statement block. For cr1, cr2, the conditions
   in switch potentially realize the above judgment. For cr0, we
   should add (free <ac->ac_g_ex.fe_len) judgment, and then delete
   (free / fragments) >= ac->ac_g_ex.fe_len), because cr0 returns
   true by default.

Signed-off-by: Chunguang Xu <brookxu@tencent.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/e20b2d8f-1154-adb7-3831-a9e11ba842e9@gmail.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>