ext4: protect journal inode's blocks using block_validity

commit 345c0dbf upstream.

Add the blocks which belong to the journal inode to block_validity's
system zone so attempts to deallocate or overwrite the journal due a
corrupted file system where the journal blocks are also claimed by
another inode.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=202879


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
[bwh: Backported to 3.16:
 - Use EXT4_HAS_COMPAT_FEATURE()
 - Use EIO instead of EFSCORRUPTED]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>