kobject: provide more diagnostic info for kobject_add_internal() failures

1/ convert open-coded KERN_ERR+dump_stack() to WARN(), so that automated
   tools pick up this warning.

2/ include the 'child' and 'parent' kobject names.  This information was
   useful for tracking down the case where scsi invoked device_del() on a
   parent object and subsequently invoked device_add() on a child.  Now the
   warning looks like:

     kobject_add_internal failed for target8:0:16 (error: -2 parent: end_device-8:0:24)
     Pid: 2942, comm: scsi_scan_8 Not tainted 3.3.0-rc7-isci+ #2
     Call Trace:
      [<ffffffff8125e551>] kobject_add_internal+0x1c1/0x1f3
      [<ffffffff81075149>] ? trace_hardirqs_on+0xd/0xf
      [<ffffffff8125e659>] kobject_add_varg+0x41/0x50
      [<ffffffff8125e723>] kobject_add+0x64/0x66
      [<ffffffff8131124b>] device_add+0x12d/0x63a
      [<ffffffff8125e0ef>] ? kobject_put+0x4c/0x50
      [<ffffffff8132f370>] scsi_sysfs_add_sdev+0x4e/0x28a
      [<ffffffff8132dce3>] do_scan_async+0x9c/0x145

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: James Bottomley <JBottomley@parallels.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>