ext4: Fix handling of extended tv_sec

In ext4, the bottom two bits of {a,c,m}time_extra are used to extend
the {a,c,m}time fields, deferring the year 2038 problem to the year
2446.

When decoding these extended fields, for times whose bottom 32 bits
would represent a negative number, sign extension causes the 64-bit
extended timestamp to be negative as well, which is not what's
intended.  This patch corrects that issue, so that the only negative
{a,c,m}times are those between 1901 and 1970 (as per 32-bit signed
timestamps).

Some older kernels might have written pre-1970 dates with 1,1 in the
extra bits.  This patch treats those incorrectly-encoded dates as
pre-1970, instead of post-2311, until kernel 4.20 is released.
Hopefully by then e2fsck will have fixed up the bad data.

Also add a comment explaining the encoding of ext4's extra {a,c,m}time
bits.

Signed-off-by: David Turner <novalis@novalis.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Mark Harris <mh8928@yahoo.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=23732
Cc: stable@vger.kernel.org