misc: zero s_jnl_blocks when adding journal online or removing external journal

misc: zero s_jnl_blocks when adding journal online or removing external journal

Erase s_jnl_blocks when removing an external journal, or adding an
internal journal online.  We can't add the backup for the internal
journal because we have no good way to get the indirect block or ETB
addresses, so the best we can do is hope that the user runs e2fsck,
which will correct that.  We are motivated to erase during external
journal removal to state emphatically that there's no journal.

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