libext2fs: place metadata blocks in the last flex_bg so they are contiguous

libext2fs: place metadata blocks in the last flex_bg so they are contiguous

Place the allocation bitmaps and inode table blocks so they are
adjacent, even in the last flexbg.

Previously, after running "mke2fs -t ext4 DEV 286720", the layout of
the last few block groups would look like this:

Group 32: (Blocks 262145-270336) [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 262145 (+0), Inode bitmap at 262161 (+16)
  Inode table at 262177-262432 (+32)
Group 33: (Blocks 270337-278528) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
  Block bitmap at 262146 (bg #32 + 1), Inode bitmap at 262162 (bg #32 + 17)
  Inode table at 262433-262688 (bg #32 + 288)
Group 34: (Blocks 278529-286719) [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 262147 (bg #32 + 2), Inode bitmap at 262163 (bg #32 + 18)
  Inode table at 262689-262944 (bg #32 + 544)

Now, they look like this:

Group 32: (Blocks 262145-270336) [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 262145 (+0), Inode bitmap at 262148 (+3)
  Inode table at 262151-262406 (+6)
Group 33: (Blocks 270337-278528) [INODE_UNINIT, BLOCK_UNINIT, ITABLE_ZEROED]
  Block bitmap at 262146 (bg #32 + 1), Inode bitmap at 262149 (bg #32 + 4)
  Inode table at 262407-262662 (bg #32 + 262)
Group 34: (Blocks 278529-286719) [INODE_UNINIT, ITABLE_ZEROED]
  Block bitmap at 262147 (bg #32 + 2), Inode bitmap at 262150 (bg #32 + 5)
  Inode table at 262663-262918 (bg #32 + 518)

This reduces the free space fragmentation in a freshly created file
system.  It also allows the following mke2fs command to succeed:

mke2fs -t ext4 -b 4096 -O ^resize_inode -G $((2**20)) DEV 2130483

(Note that while this allows people to run mke2fs with insanely large
flexbg sizes, this is not a recommended practice, as the kernel may
refuse to resize such a file system while mounted, since it currently
tries to allocate an in-memory data structure based on the size of the
flexbg, and so a file system with a very large flexbg size will cause
the memory allocation to fail.  This will hopefully be fixed in a
future kernel release, but if the goal is to force all of the metadata
blocks to be at the beginning of the file system, it's better to use
the packed_meta_blocks configuration parameter in mke2fs.conf.)

Signed-off-by: Theodore Ts'o <tytso@mit.edu>