net/rpmsg: unblock reader threads operating on errored sockets

The rpmsg_proto driver is used to provide a socket interface
to userspace under the AF_RPMSG address family, and is used
by the TI IPC MessageQ stack. The typical usage for receiving
messages include a thread blocked on a select() call with
appropriate socket fds, followed by a recvfrom() on the fd
returned/marked ready by select().

The rpmsg_sock_poll() function implements the logic needed
by the select() call, and marks a socket ready only when there
is data to be read currently. Any reader thread waiting on the
select() call to return is currently not unblocked when a remote
processor goes through an error recovery, and can remain blocked
forever as its remote processor peer thread may never send it
another message. Enhance the rpmsg_proto driver so that a waiting
thread can be unblocked by waking it up during the process of
marking the open sockets with the error status RPMSG_ERROR. This
is achieved by using the socket's .sk_error_report() ops, and is
preferred over the .sk_state_change() ops to wakeup only a single
exclusive thread.

Signed-off-by: Suman Anna <s-anna@ti.com>