tune2fs: enable uninit_bg when disabling metadata_csum

tune2fs: enable uninit_bg when disabling metadata_csum

If we're disabling metadata_csum and the user doesn't provide explicit
instructions to enable or disable uninit_bg, assume that they want
uninit_bg to be turned on by default.  Otherwise, we lose all block
group flags and unused inode count, which is a big hit to performance.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>