net: netlink: add nla_get_*_default() accessors

net: netlink: add nla_get_*_default() accessors

There are quite a number of places that use patterns
such as

  if (attr)
     val = nla_get_u16(attr);
  else
     val = DEFAULT;

Add nla_get_u16_default() and friends like that to
not have to type this out all the time.

Acked-by: Toke Høiland-Jørgensen <toke@kernel.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20241108114145.acd2aadb03ac.I3df6aac71d38a5baa1c0a03d0c7e82d4395c030e@changeid
Signed-off-by: Jakub Kicinski <kuba@kernel.org>