mke2fs: allow file systems w/ uninit_bg to be recovered with mke2fs -S

mke2fs: allow file systems w/ uninit_bg to be recovered with mke2fs -S

The command mke2fs -S is used as a last ditch recovery command to
write new superblock and block group descriptors, but _not_ to destroy
the inode table in hopes of recovering from a badly corrupted file
system.  If the uninit_bg feature is enabled, we need to make sure to
clear the unused inodes count field in the block group descriptors or
else e2fsck -fy will leave the file system completely empty.

Thanks to Akira Fujita for reporting this problem.

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