rtc: fix overflow in mktimegm

rtc: fix overflow in mktimegm

When setting a date in 1980, Linux is actually disregarding the century
byte and setting the year to 2080.  This causes a year-2038 overflow
in mktimegm.  Fix this by doing the days-to-seconds computation in
64-bit math.

Reported-by: Lucas Meneghel Rodrigues <lookkas@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit b6db4aca20e9af4f62c9c9e08b9b9672a6ed3390)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>