powerpc/pseries: Check memory device state before onlining/offlining

When DLPAR adding or removing memory we need to check the device
offline status before trying to online/offline the memory. This is
needed because calls to device_online() and device_offline() will
return non-zero for memory that is already online and offline
respectively.

This update resolves two scenarios. First, for a kernel built with
auto-online memory enabled (CONFIG_MEMORY_HOTPLUG_DEFAULT_ONLINE=y),
memory will be onlined as part of calls to add_memory(). After adding
the memory the pseries DLPAR code tries to online it and fails since
the memory is already online. The DLPAR code then tries to remove the
memory which produces the oops message below because the memory is not
offline.

The second scenario occurs when removing memory that is already
offline, i.e. marking memory offline (via sysfs) and then trying to
remove that memory. This doesn't work because offlining the already
offline memory does not succeed and the DLPAR code then fails the
DLPAR remove operation.

The fix for both scenarios is to check the device.offline status
before making the calls to device_online() or device_offline().

  kernel BUG at mm/memory_hotplug.c:1936!
  ...
  NIP [c0000000002ca428] .remove_memory+0xb8/0xc0
  LR [c0000000002ca3cc] .remove_memory+0x5c/0xc0
  Call Trace:
    .remove_memory+0x5c/0xc0 (unreliable)
    .dlpar_add_lmb+0x384/0x400
    .dlpar_memory+0x5dc/0xca0
    .handle_dlpar_errorlog+0x74/0xe0
    .pseries_hp_work_fn+0x2c/0x90
    .process_one_work+0x17c/0x460
    .worker_thread+0x88/0x500
    .kthread+0x15c/0x1a0
    .ret_from_kernel_thread+0x58/0xc0

Fixes: 943db62c

 ("powerpc/pseries: Revert 'Auto-online hotplugged memory'")
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
[mpe: Use bool, add explicit rc=0 case, change log typos & formatting]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>