powerpc/iommu: Fix the missing iommu_group_put() during platform domain attach

The function spapr_tce_platform_iommu_attach_dev() is missing to call
iommu_group_put() when the domain is already set. This refcount leak
shows up with BUG_ON() during DLPAR remove operation as:

  KernelBug: Kernel bug in state 'None': kernel BUG at arch/powerpc/platforms/pseries/iommu.c:100!
  Oops: Exception in kernel mode, sig: 5 [#1]
  LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=8192 NUMA pSeries
  <snip>
  Hardware name: IBM,9080-HEX POWER10 (raw) 0x800200 0xf000006 of:IBM,FW1060.00 (NH1060_016) hv:phyp pSeries
  NIP:  c0000000000ff4d4 LR: c0000000000ff4cc CTR: 0000000000000000
  REGS: c0000013aed5f840 TRAP: 0700   Tainted: G          I         (6.8.0-rc3-autotest-g99bd3cb0d12e)
  MSR:  8000000000029033 <SF,EE,ME,IR,DR,RI,LE>  CR: 44002402  XER: 20040000
  CFAR: c000000000a0d170 IRQMASK: 0
  ...
  NIP iommu_reconfig_notifier+0x94/0x200
  LR  iommu_reconfig_notifier+0x8c/0x200
  Call Trace:
    iommu_reconfig_notifier+0x8c/0x200 (unreliable)
    notifier_call_chain+0xb8/0x19c
    blocking_notifier_call_chain+0x64/0x98
    of_reconfig_notify+0x44/0xdc
    of_detach_node+0x78/0xb0
    ofdt_write.part.0+0x86c/0xbb8
    proc_reg_write+0xf4/0x150
    vfs_write+0xf8/0x488
    ksys_write+0x84/0x140
    system_call_exception+0x138/0x330
    system_call_vectored_common+0x15c/0x2ec

The patch adds the missing iommu_group_put() call.

Fixes: a8ca9fc9

 ("powerpc/iommu: Do not do platform domain attach atctions after probe")
Reported-by: Venkat Rao Bagalkote <venkat88@linux.vnet.ibm.com>
Closes: https://lore.kernel.org/all/274e0d2b-b5cc-475e-94e6-8427e88e271d@linux.vnet.ibm.com/
Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.vnet.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/170784021983.6249.10039296655906636112.stgit@linux.ibm.com