powerpc/pseries/svm: Fix incorrect check for shared_lppaca_size

powerpc/pseries/svm: Fix incorrect check for shared_lppaca_size

Early secure guest boot hits the below crash while booting with
vcpus numbers aligned with page boundary for PAGE size of 64k
and LPPACA size of 1k i.e 64, 128 etc.

  Partition configured for 64 cpus.
  CPU maps initialized for 1 thread per core
  ------------[ cut here ]------------
  kernel BUG at arch/powerpc/kernel/paca.c:89!
  Oops: Exception in kernel mode, sig: 5 [#1]
  LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA pSeries

This is due to the BUG_ON() for shared_lppaca_total_size equal to
shared_lppaca_size. Instead the code should only BUG_ON() if we have
exceeded the total_size, which indicates we've overflowed the array.

Fixes: bd104e6db6f0 ("powerpc/pseries/svm: Use shared memory for LPPACA structures")
Cc: stable@vger.kernel.org # v5.4+
Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: Laurent Dufour <ldufour@linux.ibm.com>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
[mpe: Reword change log to clarify we're fixing not removing the check]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200619070113.16696-1-sathnaga@linux.vnet.ibm.com