tune2fs: always check disable_uninit_bg() return code

tune2fs: always check disable_uninit_bg() return code

Enhance disable_uninit_bg() to return error codes -- if something goes
wrong, we want to flag the FS as needing a fsck and exit.  Mr. Reardon
discovered that tune2fs -O ^metadata_csum on a FS with a corrupt
bitmap would leave the FS in a weird state.

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