fs/hpfs: extend gmt_to_local() conversion to 64-bit times

The VFS timestamps are all 64-bit now, the only missing piece for hpfs
is the internal conversion function.  One interesting bit about hpfs is
that it can already deal with moving the 136 year window of its
timestamps to support a much wider range than other file systems with
32-bit timestamps.  It also treats the timestamps as 'unsigned' on
64-bit architectures (but signed on 32-bit, because time_t always around
to negative numbers in 2038).

Changing the conversion to use time64_t makes 32-bit architectures
behave the same way as 64-bit.  For completeness, this also adds a
clamp_t call for each conversion, so we don't wrap the timestamps but
instead stay within the [0..U32_MAX] range of the on-disk timestamps.

Link: http://lkml.kernel.org/r/20180718115017.742609-3-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>