s390/qeth: fix IP address lookup for L3 devices

Current code ("qeth_l3_ip_from_hash()") matches a queried address object
against objects in the IP table by IP address, Mask/Prefix Length and
MAC address ("qeth_l3_ipaddrs_is_equal()"). But what callers actually
require is either
a) "is this IP address registered" (ie. match by IP address only),
before adding a new address.
b) or "is this address object registered" (ie. match all relevant
   attributes), before deleting an address.

Right now
1. the ADD path is too strict in its lookup, and eg. doesn't detect
conflicts between an existing NORMAL address and a new VIPA address
(because the NORMAL address will have mask != 0, while VIPA has
a mask == 0),
2. the DELETE path is not strict enough, and eg. allows del_rxip() to
delete a VIPA address as long as the IP address matches.

Fix all this by adding helpers (_addr_match_ip() and _addr_match_all())
that do the appropriate checking.

Note that the ADD path for NORMAL addresses is special, as qeth keeps
track of how many times such an address is in use (and there is no
immediate way of returning errors to the caller). So when a requested
NORMAL address _fully_ matches an existing one, it's not considered a
conflict and we merely increment the refcount.

Fixes: 5f78e29c

 ("qeth: optimize IP handling in rx_mode callback")
Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>