fs/9p: avoid accessing utsname after namespace has been torn down

During trinity fuzzing in a kvmtool guest, I stumbled across the
following:

Unable to handle kernel NULL pointer dereference at virtual address 00000004
PC is at v9fs_file_do_lock+0xc8/0x1a0
LR is at v9fs_file_do_lock+0x48/0x1a0
[<c01e2ed0>] (v9fs_file_do_lock+0xc8/0x1a0) from [<c0119154>] (locks_remove_flock+0x8c/0x124)
[<c0119154>] (locks_remove_flock+0x8c/0x124) from [<c00d9bf0>] (__fput+0x58/0x1e4)
[<c00d9bf0>] (__fput+0x58/0x1e4) from [<c0044340>] (task_work_run+0xac/0xe8)
[<c0044340>] (task_work_run+0xac/0xe8) from [<c002e36c>] (do_exit+0x6bc/0x8d8)
[<c002e36c>] (do_exit+0x6bc/0x8d8) from [<c002e674>] (do_group_exit+0x3c/0xb0)
[<c002e674>] (do_group_exit+0x3c/0xb0) from [<c002e6f8>] (__wake_up_parent+0x0/0x18)

I believe this is due to an attempt to access utsname()->nodename, after
exit_task_namespaces() has been called, leaving current->nsproxy->uts_ns
as NULL and causing the above dereference.

A similar issue was fixed for lockd in 9a1b6bf818 ("LOCKD: Don't call
utsname()->nodename from nlmclnt_setlockargs"), so this patch attempts
something similar for 9pfs.

Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

include/net/9p/client.h

@@ -26,6 +26,8 @@
 #ifndef NET_9P_CLIENT_H
 #define NET_9P_CLIENT_H
 
+#include <linux/utsname.h>
+
 /* Number of requests per row */
 #define P9_ROW_MAXTAG 255
 
@@ -134,6 +136,7 @@ struct p9_req_t {
  * @tagpool - transaction id accounting for session
  * @reqs - 2D array of requests
  * @max_tag - current maximum tag id allocated
+ * @name - node name used as client id
  *
  * The client structure is used to keep track of various per-client
  * state that has been instantiated.
@@ -164,6 +167,8 @@ struct p9_client {
 	struct p9_idpool *tagpool;
 	struct p9_req_t *reqs[P9_ROW_MAXTAG];
 	int max_tag;
+
+	char name[__NEW_UTS_LEN + 1];
 };
 
 /**