AF_VSOCK: Shrink the area influenced by prepare_to_wait

AF_VSOCK: Shrink the area influenced by prepare_to_wait

commit f7f9b5e7f8eccfd68ffa7b8d74b07c478bb9e7f0 upstream.

When a thread is prepared for waiting by calling prepare_to_wait, sleeping
is not allowed until either the wait has taken place or finish_wait has
been called.  The existing code in af_vsock imposed unnecessary no-sleep
assumptions to a broad list of backend functions.
This patch shrinks the influence of prepare_to_wait to the area where it
is strictly needed, therefore relaxing the no-sleep restriction there.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: "Jorgen S. Hansen" <jhansen@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>