Fix IpConversionUtil.extractIpv4AddressMask()
The algorithm here is wrong when prefix length is 0, as it
returns 255.255.255.255, where it should be returning 0.0.0.0.
This turns out to be an error coming from the fact we use a long
bitmask and incorrect masks to mask out the topmost long bits.
Use an int bitmask and switch to non-sign-extending shifts, which
fixes the problem.
JIRA: OPNFLWPLUG-1063
Change-Id: Idd42ba510a3a9d2032646f07b0bde8061ecd2637
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>