pkt_sched: Change PSCHED_SHIFT from 10 to 6

Change PSCHED_SHIFT from 10 to 6 to increase schedulers time
resolution. This will increase 16x a number of (internal) ticks per
nanosecond, and is needed to improve accuracy of schedulers based on
rate tables, like HTB, TBF or CBQ, with rates above 100Mbit. It is
assumed this change is safe for 32bit accounting of time diffs up
to 2 minutes, which should be enough for common use (extremely low
rate values may overflow, so get inaccurate instead). To make full
use of this change an updated iproute2 will be needed. (But using
older iproute2 should be safe too.)

This change breaks ticks - microseconds similarity, so some minor code
fixes might be needed. It is also planned to change naming adequately
eg. to PSCHED_TICKS2NS() etc. in the near future.

Reported-by: Antonio Almeida <vexwek@gmail.com>
Tested-by: Antonio Almeida <vexwek@gmail.com>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>