net: 9p: fix refcount leak in p9_read_work() error handling

p9_req_put need to be called when m->rreq->rc.sdata is NULL to avoid
temporary refcount leak.

Link: https://lkml.kernel.org/r/20220712104438.30800-1-hbh25y@gmail.com
Fixes: 728356dede ("9p: Add refcount to p9_req_t")
Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
[Dominique: commit wording adjustments, p9_req_put argument fixes for rebase]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/trans_fd.c

@@ -343,6 +343,7 @@ static void p9_read_work(struct work_struct *work)
 			p9_debug(P9_DEBUG_ERROR,
 				 "No recv fcall for tag %d (req %p), disconnecting!\n",
 				 m->rc.tag, m->rreq);
+			p9_req_put(m->client, m->rreq);
 			m->rreq = NULL;
 			err = -EIO;
 			goto error;