Add support for new compat feature "sparse_super2"

Add support for new compat feature "sparse_super2"

In practice, it is **extremely** rare for users to try to use more
than the first backup superblock located at the beginning of block
group #1.  (i.e., at block number 32768 for file systems with a 4k
block size).  This new compat feature restricts the backup superblock
to block group #1 and the last block group in the file system.

Aside from reducing the overhead of the file system by a small number
of blocks, by eliminating the rest of the backup superblocks, it
allows us to have a much more flexible metadata layout.  For example,
we can force all of the allocation bitmaps and inode table blocks to
the beginning of the disk, which allows most of the disk to be
exclusively used for contiguous data blocks.

This simplifies taking advantage of certain HDD specific features,
such as Shingled Magnetic Recording (aka Shingled Drives), and the
TCG's OPAL Storage Specification where having a simple mapping between
LBA block ranges and the data blocks used by the file system can make
life much simpler.

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