staging/lustre: use 64-bit time for pl_recalc

The ldlm pool calculates elapsed time by comparing the previous and
current get_seconds() values, which is unsafe on 32-bit machines
after 2038.

This changes the code to use time64_t and ktime_get_real_seconds(),
keeping the 'real' instead of 'monotonic' time because of the
debug prints.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>