mke2fs: prevent creation of unmountable ext4 with large flex_bg count

mke2fs: prevent creation of unmountable ext4 with large flex_bg count

In mke2fs command, if flex_bg count is too large to filesystem blocks
count, unmountable ext4 which has the out of filesystem block offset
is created (Case1).  Moreover this large flex_bg count causes an
unintentional metadata layout (bmap-imap-itable-bmap-imap-itable .. in
block group) (Case2).

To fix these issues and keep healthy flex_bg layout, disallow creating
ext4 with obviously large flex_bg count to filesystem blocks count.

Steps to reproduce:
(Case1)
1.
    # mke2fs -t ext4 -b 4096 -O ^resize_inode -G $((2**20)) DEV 2130483

2.
    # mount -t ext4 DEV MP
    mount: wrong fs type, bad option, bad superblock on /dev/sdb4,
           missing codepage or helper program, or other error
           In some cases useful info is found in syslog - try
           dmesg | tail  or so

3.
    # dumpe2fs DEV
    ...
    Block count:              2130483
    ...
    Flex block group size:    1048576
    ...
    Group 65: (Blocks 2129920-2130482) [INODE_UNINIT]
      Checksum 0x4cb3, unused inodes 8080
      Block bitmap at 67 (bg #0 + 67), Inode bitmap at 1048643 (bg #32 + 67)
      Inode table at 2129979-2130483 (+59)
                             ^^^^^^^  2130483 is out of FS!
      65535 free blocks, 8080 free inodes, 0 directories, 8080 unused inodes
      Free blocks:
      Free inodes: 525201-533280

(Case2)
1.
    # mke2fs -t ext4 -G 2147483648 DEV 3145728

2.
    # debugfs -R stats DEV
    ...
    Block count:              786432
    ...
    Flex block group size:    2147483648
    ...
     Group  0: block bitmap at 193, inode bitmap at 194, inode table at 195
    ...
     Group  1: block bitmap at 707, inode bitmap at 708, inode table at 709
    ...
     Group  2: block bitmap at 1221, inode bitmap at 1222, inode table at 1223
    ...

Signed-off-by: Akira Fujita <a-fujita@rs.jp.nec.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>