modules: proper cleanup of kobject without CONFIG_SYSFS

kobject: '<NULL>' (ffffffffa0104050): is not initialized, yet kobject_put() is being called.
------------[ cut here ]------------
WARNING: at /home/den/src/linux-netns26/lib/kobject.c:583 kobject_put+0x53/0x55()
Modules linked in: ipv6 nfsd lockd nfs_acl auth_rpcgss sunrpc exportfs ide_cd_mod cdrom button [last unloaded: pktgen]
comm: rmmod Tainted: G        W 2.6.26-rc3 #585
Call Trace:
  [<ffffffff802359ab>] warn_on_slowpath+0x58/0x7a
  [<ffffffff80236aca>] ? printk+0x67/0x69
  [<ffffffff80236aca>] ? printk+0x67/0x69
  [<ffffffff80324289>] kobject_put+0x53/0x55
  [<ffffffff8025e2ee>] free_module+0x87/0xfa
  [<ffffffff8025fee5>] sys_delete_module+0x178/0x1e1
  [<ffffffff804b1e70>] ? lockdep_sys_exit_thunk+0x35/0x67
  [<ffffffff804b1dff>] ? trace_hardirqs_on_thunk+0x35/0x3a
  [<ffffffff8020c0bb>] system_call_after_swapgs+0x7b/0x80
---[ end trace 8f5aafa7f6406cf8 ]---

mod->mkobj.kobj is not initialized without CONFIG_SYSFS. Do not call
kobject_put in this case.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>