netconsole: avoid a crash with multiple sysfs writers

When my 'ifup eth' script was fired multiple times and ran concurrent on
my laptop, for some obscure /etc scripting reason, it was revealed
that the store_enabled() function in netconsole doesn't handle it nicely,
as recorded by the Oops below (a syslog paste, but not mangled too much
to prevent from discerning the traceback).

On Linux 3.10.4, this patch seeks to remedy the problem, and it has been
running stable on my laptop for a few days.

[52608.609325] BUG: unable to handle kernel NULL pointer dereference at 00000000000003e0
[52608.609331] IP: [<ffffffff81532a17>] __netpoll_cleanup+0x27/0xe0
[52608.609339] PGD 15e51a067 PUD 15433e067 PMD 0
[52608.609343] Oops: 0000 [#1] SMP re firewire_ohci firewire_core crc_itu_t [last unloaded: kvm_intel]
[52608.609347] Modules linked in: kvm_intel tun vfat fat ppdev parport_pc parport fuse ipt_MASQUERADE usb_storage nf_conntrack_netbios_ns nf_conn [..garbled..]
[52608.609433] RAX: 0000000000000000 RBX: ffff880210bbcc68 RCX: 0000000000000000
[52608.609435] RDX: 0000000000000000 RSI: ffff8801ba447da0 RDI: ffff880210bbcc68
[52608.609437] RBP: ffff8801ba447e18 R08: 0000000000000000 R09: 0000000000000001
[52608.609439] R10: 000000000000000a R11: f000000000000000 R12: ffff880210bbcc68
[52608.609441] R13: ffff88020bc41000 R14: 0000000000000002 R15: 000000000000000200000000000
[52608.609443] FS:  00007f38d7bff740(0000) GS:ffff88021dc40000(0000) knlGS:0000000000000000
[52608.609446] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003300000000001427e0
[52608.609448] CR2: 00000000000003e0 CR3: 0000000154103000 CR4: 00000000001427e0
[52608.609450] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[52608.609452] netpoll: netconsole: local port 6665ess 10.0.0.27
[52608.609454] netpoll: netconsole: local IPv4 address 10.0.0.27
[52608.609456] netpoll: netconsole: interface 'em1'
[52608.609457] netpoll: netconsole: remote port 514ress 10.0.0.15
[52608.609459] netpoll: netconsole: remote IPv4 address 10.0.0.15:65:a8:9a:c7
[52608.609461] netpoll: netconsole: remote ethernet address 1c:6f:65:a8:9a:c7
[52608.609463] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[52608.609464] Stack:801ba447e08 ffff880210bbcc68 ffffffffffffffea ffff88020bc41000
[52608.609466]  ffff8801ba447e08 ffff880210bbcc68 ffffffffffffffea ffff88020bc41000
[52608.609471]  0000000000000002 0000000000000002 ffff8801ba447e38 ffffffff81532af4
[52608.609475]  0000000000000000 ffff880210bbcc00 ffff8801ba447e78 ffffffff81420e7c
[52608.609479] Call Trace:
[52608.609484]  [<ffffffff81532af4>] netpoll_cleanup+0x24/0x50
[52608.609489]  [<ffffffff81420e7c>] store_enabled+0x5c/0xe0
[52608.609492]  [<ffffffff81420abe>] netconsole_target_attr_store+0x2e/0x40
[52608.609498]  [<ffffffff811ff2a2>] configfs_write_file+0xd2/0x130
[52608.609503]  [<ffffffff81188f95>] vfs_write+0xc5/0x1f0
[52608.609506]  [<ffffffff81189482>] SyS_write+0x52/0xa0/0x10
[52608.609511]  [<ffffffff81628c2e>] ? do_page_fault+0xe/0x10
[52608.609516]  [<ffffffff8162d402>] system_call_fastpath+0x16/0x1b
[52608.609517] Code: 1f 44 00 00 0f 1f 44 00 00 55 48 89 e5 48 83 ec 30 4c 89 65 e0 48 89 5d d8 49 89 fc 4c 89 6d e8 4c 89 75 f0 4c 89 7d f8 48 8 [..garbled..]
[52608.609559] RIP  [<ffffffff81532a17>] __netpoll_cleanup+0x27/0xe0
[52608.609563]  RSP <ffff8801ba447de8>
[52608.609564] CR2: 00000000000003e0
[52608.609567] ---[ end trace d25ec343349b61d2 ]---

Signed-off-by: Dan Aloni <alonid@postram.com>
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>