netlink: Fix printing of zero-length prefixes

netlink: Fix printing of zero-length prefixes

When delinearizing, an all-zero mask didn't qualify as prefix. Therefore
a statement:

| ip daddr 0.0.0.0/0

would be printed as:

| ip daddr & 0.0.0.0 == 0.0.0.0

To fix this, expr_mask_is_prefix() must return true if the initial 1-bit
search fails (the given value must be zero in this case). Additionally,
a shortcut is needed in conversion algorithm of expr_mask_to_prefix()
to not turn the zero prefix into a 1 by accident.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>