unix/dgram: inline sbappendaddr_locked() into uipc_sosend_dgram()

[glebius]

Summary

This allows to remove one M_NOWAIT allocation and also makes it
more clear what's going on.

[glebius]

Note: this patch is a step towards D35303.

[markj]

sys/kern/uipc_usrreq.c

1169

Why NOWAIT?

[glebius]

sys/kern/uipc_usrreq.c

1169

Because in most cases a kernel thread can't sleep here.

[markj]

sys/kern/uipc_usrreq.c

1187

I would assert from->sa_len <= MLEN here. Should be ok because sizeof(struct sockaddr_un) is smaller than MLEN, but that could be false in some exotic situations. (Sometimes one adds extra fields to struct mbuf for debugging purposes.)

[glebius]

Rebase

[glebius]

Rebase, variable rename. No functional changes.