xfs/424: add check for finobt && update comments

xfs/424: add check for finobt && update comments

1) This test can check if setting types causes error regardless of
   supporting crc, so we can update existed comments about it.  We
   also add new comments about known issues triggered in this test.

2) When finobt is disabled, xfs_db fails to get current address of
   free_root, as below:
   xfs_db -c "agi" -c "addr free_root" -c "daddr" /dev/sda11
   Metadata CRC error detected at xfs_inobt block 0x0/0x1000
   ...
   Running related tests without finobt makes no sense, so we add
   check for finobt.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>