SUNRPC: Fix an Oops in udp_poll()

udp_poll() checks the struct file for the O_NONBLOCK flag, so we must not
call it with a NULL file pointer.

Fixes: 0ffe86f4 ("SUNRPC: Use poll() to fix up the socket requeue races")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>