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Block groups that have no used inodes have their inode table left
uninitialized, unless -t is specified, in which case they are
explicitly zeroed.  When they are uninitialized, writing a sparse
ext4 image over existing data can cause e2fsck to confuse the
uninitialized data for lost inodes.

Set the EXT4_BG_INODE_UNINIT flags on block groups that have no
used inodes.  This flag requires the block group checksum feature
to be enabled, so also enable the checksum feature in the superblock
and compute the checksum for the block group.

Since zeroing the inode tables is now useless, remove the code for
it and deprecate the -t command line option.

Change-Id: I4927c1d866d051547cf0dadc8c8703ded0163925