Some subsystems dont need more than 32bits timestamps.

See for example net/ipv4/inetpeer.c and include/net/tcp.h :
#define tcp_time_stamp            ((__u32)(jiffies))


Because most timeouts should work with 'normal jiffies' that are 32bits on 
32bits platforms, it makes sense to be able to use only 32bits to store them 
and not 64 bits, to save ram.

This patch introduces jiffies_32, and related comparison functions 
time_after32(), time_before32(), time_after_eq32() and time_before_eq32().

I plan to use this infrastructure in network code for example (struct 
dst_entry comes to mind).

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>