util: Add be32_prefix_mask().

Shifting a 32-bit entity by 32 bits is undefined behavior.  As we have 2
cases where we may hit this, it is a time to introduce a helper for
this.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>