svcrpc: no need for XPT_DEAD check in svc_xprt_enqueue

If any xprt marked DEAD is also left BUSY for the rest of its life, then
the XPT_DEAD check here is superfluous--we'll get the same result from
the XPT_BUSY check just after.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>