jbd2: fix r_count overflows leading to buffer overflow in journal recovery

jbd2: fix r_count overflows leading to buffer overflow in journal recovery

commit e531d0bceb402e643a4499de40dd3fa39d8d2e43 upstream.

The journal revoke block recovery code does not check r_count for
sanity, which means that an evil value of r_count could result in
the kernel reading off the end of the revoke table and into whatever
garbage lies beyond.  This could crash the kernel, so fix that.

However, in testing this fix, I discovered that the code to write
out the revoke tables also was not correctly checking to see if the
block was full -- the current offset check is fine so long as the
revoke table space size is a multiple of the record size, but this
is not true when either journal_csum_v[23] are set.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>
[bwh: Backported to 3.2: journal checksumming is not supported, so only
 the first fix is needed]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>