datapath: Change u64_stats_* to use _irq instead of _bh().

The upstream u64_stats API has been changed to remove the _bh()
versions and switch all consumers to use IRQ safe variants instead.
This was done to be safe for netpoll generated packets, which can
occur in hard IRQ context. From a safety perspective, this doesn't
directly affect OVS since it doesn't support netpoll. However, this
change has been backported to older kernels so OVS needs to use the
new API to compile.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Pritesh Kothari <pritesh.kothari@cisco.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>