[PATCH] Add getnstimestamp function

There are several functions that might seem appropriate for a timestamp:

get_cycles()
current_kernel_time()
do_gettimeofday()
<read jiffies/jiffies_64>

Each has problems with combinations of SMP-safety, low resolution, and
monotonicity. This patch adds a new function that returns a monotonic SMP-safe
timestamp with nanosecond resolution where available.

Changes:
	Split timestamp into separate patch
	Moved to kernel/time.c
	Renamed to getnstimestamp
	Fixed unintended-pointer-arithmetic bug

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>