af_unix: Copy unix_mkname() into unix_find_(bsd|abstract)().

We should not call unix_mkname() before unix_find_other() and instead do
the same thing where necessary based on the address type:

  - terminating the address with '\0' in unix_find_bsd()
  - calculating the hash in unix_find_abstract().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>