Fix a resource leak in doaccept().
If a connection that is being accepted gets aborted early, or if the
user-supplied buffer is invalid, doaccept() leaks a socket. This is
a regression caused by r1.153 of uipc_syscalls.c.
Correct the issue by associating the socket with the file early enough.
In case soaccept() or copyaddrout() fails, the socket will be freed
as a result of the file closing. This logic was used by the pre-r1.153
closef() may block, so it is hoisted outside the fdp lock.
OK bluhm@ mpi@