nfsd: initialize the exp->ex_uuid field in svc_export_init

nfsd: initialize the exp->ex_uuid field in svc_export_init

commit 885c91f7466 in Bruce's tree was causing oopses for me:

general protection fault: 0000 [#1] SMP
Modules linked in: nfsd(OF) nfs_acl(OF) auth_rpcgss(OF) lockd(OF) sunrpc(OF) kvm_amd kvm microcode i2c_piix4 virtio_net virtio_balloon cirrus drm_kms_helper ttm drm virtio_blk i2c_core
CPU 0
Pid: 564, comm: exportfs Tainted: GF          O 3.8.0-0.rc5.git2.1.fc19.x86_64 #1 Bochs Bochs
RIP: 0010:[<ffffffff811b1509>]  [<ffffffff811b1509>] kfree+0x49/0x280
RSP: 0018:ffff88007a3d7c50  EFLAGS: 00010203
RAX: 01adaf8dadadad80 RBX: 6b6b6b6b6b6b6b6b RCX: 0000000000000001
RDX: ffffffff7fffffff RSI: 0000000000000000 RDI: 6b6b6b6b6b6b6b6b
RBP: ffff88007a3d7c80 R08: 6b6b6b6b6b6b6b6b R09: 0000000000000000
R10: 0000000000000018 R11: 0000000000000000 R12: ffff88006a117b50
R13: ffffffffa01a589c R14: ffff8800631b0f50 R15: 01ad998dadadad80
FS:  00007fcaa3616740(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
CR2: 00007f5d84b6fdd8 CR3: 0000000064db4000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process exportfs (pid: 564, threadinfo ffff88007a3d6000, task ffff88006af28000)
Stack:
 ffff88007a3d7c80 ffff88006a117b68 ffff88006a117b50 0000000000000000
 ffff8800631b0f50 ffff88006a117b50 ffff88007a3d7ca0 ffffffffa01a589c
 ffff880036be1148 ffff88007a3d7cf8 ffff88007a3d7e28 ffffffffa01a6a98
Call Trace:
 [<ffffffffa01a589c>] svc_export_put+0x5c/0x70 [nfsd]
 [<ffffffffa01a6a98>] svc_export_parse+0x328/0x7e0 [nfsd]
 [<ffffffffa016f1c7>] cache_do_downcall+0x57/0x70 [sunrpc]
 [<ffffffffa016f25e>] cache_downcall+0x7e/0x100 [sunrpc]
 [<ffffffffa016f338>] cache_write_procfs+0x58/0x90 [sunrpc]
 [<ffffffffa016f2e0>] ? cache_downcall+0x100/0x100 [sunrpc]
 [<ffffffff8123b0e5>] proc_reg_write+0x75/0xb0
 [<ffffffff811ccecf>] vfs_write+0x9f/0x170
 [<ffffffff811cd089>] sys_write+0x49/0xa0
 [<ffffffff816e0919>] system_call_fastpath+0x16/0x1b
Code: 66 66 66 90 48 83 fb 10 0f 86 c3 00 00 00 48 89 df 49 bf 00 00 00 00 00 ea ff ff e8 f2 12 ea ff 48 c1 e8 0c 48 c1 e0 06 49 01 c7 <49> 8b 07 f6 c4 80 0f 85 1d 02 00 00 49 8b 07 a8 80 0f 84 ee 01
RIP  [<ffffffff811b1509>] kfree+0x49/0x280
 RSP <ffff88007a3d7c50>

I think Majianpeng's patch is correct, but incomplete. In order for it
to be safe to free the ex_uuid unconditionally in svc_export_put, we
need to make sure it's initialized to NULL in the init routine.

Cc: majianpeng <majianpeng@gmail.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>