SUNRPC: fix a list corruption issue in xprt_release()

We remove the request from the receive list before we call
xprt_wait_on_pinned_rqst(), and so we need to use list_del_init().
Otherwise, we will see list corruption when xprt_complete_rqst()
is called.

Reported-by: Emre Celebi <emre@primarydata.com>
Fixes: ce7c252a ("SUNRPC: Add a separate spinlock to protect...")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>