NFS: use TCP for MOUNT when xprtsec= selects TLS

An NFSv2 or NFSv3 text mount that specifies xprtsec= but no
mountproto= crashes in nfs_init_timeout_values():

  RIP: 0010:nfs_init_timeout_values+0x10a/0x110 [nfs]
  Call Trace:
   nfs_mount+0x1c3/0x330 [nfs]
   nfs_try_get_tree+0x1e5/0x460 [nfs]
   nfs_get_tree+0x64d/0x6d0 [nfs]
   vfs_get_tree+0x2b/0xe0
   vfs_cmd_create+0x5f/0xd0
   vfs_fsconfig_locked+0x50/0x130
   __se_sys_fsconfig+0x2f9/0x3a0

nfs_validate_transport_protocol() sets nfs_server.protocol to
XPRT_TRANSPORT_TCP_TLS when xprtsec= is present, but the switch in
nfs_set_mount_transport_protocol() has no case for that value.
mount_server.protocol stays zero, and nfs_init_timeout_values()
calls BUG() on a transport it does not recognize.

mount.nfs always appends mountproto= after probing the server's
rpcbind, so only a caller that bypasses it, such as "mount -i" or a
program that drives fsconfig() directly, reaches the crash.

Select TCP for MOUNT when the NFS transport is XPRT_TRANSPORT_TCP_TLS.
nfs_mount() requests no transport security, so MOUNT already runs
over plain TCP whenever mount.nfs supplies mountproto=tcp.

Fixes: c8407f2e56 ("NFS: Add an "xprtsec=" NFS mount option")
Signed-off-by: Chuck Lever <cel@kernel.org>
Reviewed-by: Prabhakar Pujeri <prabhakar.pujeri@dell.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/fs_context.c

@@ -402,6 +402,7 @@ static void nfs_set_mount_transport_protocol(struct nfs_fs_context *ctx)
 		ctx->mount_server.protocol = XPRT_TRANSPORT_UDP;
 		break;
 	case XPRT_TRANSPORT_TCP:
+	case XPRT_TRANSPORT_TCP_TLS:
 	case XPRT_TRANSPORT_RDMA:
 		ctx->mount_server.protocol = XPRT_TRANSPORT_TCP;
 	}