af_unix: speedup /proc/net/unix /proc/net/unix has quadratic behavior, and can hold unix_table_lock for a while if high number of unix sockets are alive. (90 ms for 200k sockets...)

We already have a hash table, so its quite easy to use it.

Problem is unbound sockets are still hashed in a single hash slot
(unix_socket_table[UNIX_HASH_TABLE])

This patch also spreads unbound sockets to 256 hash slots, to speedup
both /proc/net/unix and unix_diag.

Time to read /proc/net/unix with 200k unix sockets :
(time dd if=/proc/net/unix of=/dev/null bs=4k)

before : 520 secs
after : 2 secs

idea is taken here - https://github.com/flar2/flo/commit/ef972b303cbbf813f8bc2228e34c7fed9ed85ab0