acl: Avoid undefined behaviour due to 32-bit shift.

acl: Avoid undefined behaviour due to 32-bit shift.

On x86 a shift by 32-bits is a nop, but this is not backed by any
standard. Moreover, not shifting at all results in an effective netmask
of 32, which is unexpected for for v4_addr_match(), as the function
is supposed to answer the question "do the first zero bits of these
two addresses match?". This should always be true, regardless of the
two addresses.

Fix this, and change the return type of the function while at it.