9p/fd: Use P9_HDRSZ for header size

Cleanup hardcoded header sizes to use P9_HDRSZ instead of '7'

Link: https://lkml.kernel.org/r/20221117091159.31533-4-guozihua@huawei.comSigned-off-by: GUO Zihua <guozihua@huawei.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
[Dominique: commit message adjusted to make sense after offset size
adjustment got removed]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

net/9p/trans_fd.c

@@ -120,7 +120,7 @@ struct p9_conn {
 	struct list_head unsent_req_list;
 	struct p9_req_t *rreq;
 	struct p9_req_t *wreq;
-	char tmp_buf[7];
+	char tmp_buf[P9_HDRSZ];
 	struct p9_fcall rc;
 	int wpos;
 	int wsize;
@@ -293,7 +293,7 @@ static void p9_read_work(struct work_struct *work)
 	if (!m->rc.sdata) {
 		m->rc.sdata = m->tmp_buf;
 		m->rc.offset = 0;
-		m->rc.capacity = 7; /* start by reading header */
+		m->rc.capacity = P9_HDRSZ; /* start by reading header */
 	}
 
 	clear_bit(Rpending, &m->wsched);
@@ -316,7 +316,7 @@ static void p9_read_work(struct work_struct *work)
 		p9_debug(P9_DEBUG_TRANS, "got new header\n");
 
 		/* Header size */
-		m->rc.size = 7;
+		m->rc.size = P9_HDRSZ;
 		err = p9_parse_header(&m->rc, &m->rc.size, NULL, NULL, 0);
 		if (err) {
 			p9_debug(P9_DEBUG_ERROR,