fix v5.0-rc1, SUNRPC: init machine_cred.magic under CONFIG_DEBUG_CREDENTIALS

[J. R. Okajima]

By the commit,
a52458b48af1 2018-12-19 NFS/NFSD/SUNRPC: replace generic creds with 'struct cred'.

struct rpc_cred machine_cred was converted to struct cred, but
machine_cred.magic is still uninitialized.  Without initializing it, I
got 'Invalid credentials' error.  It is necessary when
CONFIG_DEBUG_CREDENTIALS is enabled.

CRED: Invalid credentials
CRED: At /proj/aufs/aufs4-linux.git/include/linux/cred.h:253
CRED: Specified credentials: 000000006ca067d8 
CRED: ->magic=0, put_addr=          (null)
CRED: ->usage=1, subscr=0
CRED: ->*uid = { 0,0,0,0 }
CRED: ->*gid = { 0,0,0,0 }
CRED: ->security is           (null)
------------[ cut here ]------------
kernel BUG at /proj/aufs/aufs4-linux.git/kernel/cred.c:825!
invalid opcode: 0000 [#1] PREEMPT SMP PTI
CPU: 0 PID: 24923 Comm: mount.nfs4 Tainted: G        W         5.0.0-rc1aufsD+ #906
Hardware name: Pegatron Pegatron/IPM41, BIOS 0001 02/05/2009
RIP: 0010:__invalid_creds+0x4d/0x60
Code: 44 89 ea 4c 89 e6 48 c7 c7 cf 92 49 82 e8 5e 21 05 00 48 c7 c6 e1 92 49 82 48 89 df 65 48 8b 14 25 80 4e 01 00 e8 23 fe ff ff <0f> 0b 48 c7 c7 00 ac 68 82 e8 25 6a 53 00 0f 1f 44 00 00 66 66 66
RSP: 0018:ffff88810bedf918 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffffffff829016c0 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff8294c428
RBP: ffff88810bedf930 R08: 0000000000000001 R09: 0000000000000000
R10: ffff88810bedf930 R11: 0000000000000000 R12: ffffffff824964d0
R13: 00000000000000fd R14: ffffffff829016c0 R15: 0000000000000001
FS:  00007f8616b55480(0000) GS:ffff88811ba00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fda79fe7170 CR3: 00000000aca90000 CR4: 00000000000406f0
Call Trace:
 nfs4_discover_server_trunking+0x2cb/0x330
 nfs4_init_client+0x16e/0x210
 ? lockdep_init_map+0x57/0x1d0
 ? rpc_wake_up_task_on_wq_queue_action_locked+0x60/0x60
 ? nfs_get_client+0x500/0x680
 nfs_get_client+0x51d/0x680
 nfs4_set_client+0xb9/0x130
 nfs4_create_server+0x10d/0x290
 nfs4_remote_mount+0x30/0x90
 mount_fs+0x51/0x220
 ? __init_waitqueue_head+0x3b/0x50
 vfs_kern_mount+0x6b/0x190
 ? nfs_do_root_mount+0x3c/0xc0
 nfs_do_root_mount+0x84/0xc0
 nfs4_try_mount+0x37/0x50
 nfs_fs_mount+0x2a1/0xa40
 ? nfs_clone_super+0x80/0x80
 ? nfs_free_parsed_mount_data+0x60/0x60
 mount_fs+0x51/0x220
 ? nfs_alloc_parsed_mount_data+0xd0/0xd0
 ? mount_fs+0x51/0x220
 ? __init_waitqueue_head+0x3b/0x50
 vfs_kern_mount+0x6b/0x190
 ? ns_capable_common+0xc3/0x110
 do_mount+0x220/0xf90
 ksys_mount+0xea/0x170
 __x64_sys_mount+0x25/0x30
 do_syscall_64+0x79/0x360
 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Here is a patch.

diff --git a/net/sunrpc/auth.c b/net/sunrpc/auth.c
index 1ff9768f5456..0e5236a3b5e0 100644
--- a/net/sunrpc/auth.c
+++ b/net/sunrpc/auth.c
@@ -41,6 +41,9 @@ static unsigned long number_cred_unused;
 
 static struct cred machine_cred = {
 	.usage = ATOMIC_INIT(1),
+#ifdef CONFIG_DEBUG_CREDENTIALS
+	.magic = CRED_MAGIC
+#endif
 };
 
 /*