dm-latency: use ktime_get() for time stamp

Previous patch uses sched_clock() to get time stamp for BIO latency,
which might be problematic considering that sched_lock() returns per-cpu
tsc counter.

A bio might be completed on another CPU from the CPU where the bio was
issued. In this case, the difference between different per-cpu tsc
counter as IO latency will be inaccurate.

This patch replaces tsc by hrtimer (access hrtimer by ktime_get()) as a
global time stamp source, by this fix, the io-latency calculation should
be accurante.

This patch also removes ns2usecs() routine, since an exsiting
in-kernel routine ktime_to_us() is used now.

Because ktime_get() returns s64 value type, member "start_time_usec" of
struct dm_io is changed to unsigned long from unsigned long long as
well. If counter inside ktime_get() overflows and gets wrapped (which
should be very rare for 64bits counter), we just ignore this I/O
latency.
dm-latency: include <asm/atomic.h>

Previous patch includes <linux/atomic.h>, in latest redhat 2.6.32
kernel, the correct header location changes to <asm/atomic.h>.

This patch updates atomic.h including path.
dm-latency: remove useless debugging code

There are 2 (currently) useless debugging lines (printk called by
dm_enter()) in 0005-dm-latency-add-reset-function-in-sysfs-interface.patch.

This patch removes these 2 useless debugging lines.