resources: add interfaces that return conflict information

request_resource() and insert_resource() only return success or failure,
which no information about what existing resource conflicted with the
proposed new reservation.  This patch adds request_resource_conflict()
and insert_resource_conflict(), which return the conflicting resource.

Callers may use this for better error messages or to adjust the new
resource and retry the request.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>