powerpc/book3s64/radix: Remove WARN_ON in destroy_context()

On failed task initialization due to memory allocation failures, we can
call into destroy_context() with process_tb entry already populated.
This patch forces the process_tb entry to zero in destroy_context().
With this patch, we lose the ability to track if we are destroying a
context without flushing the process table entry.

  WARNING: CPU: 4 PID: 6368 at arch/powerpc/mm/mmu_context_book3s64.c:246 destroy_context+0x58/0x340
  NIP [c0000000000875f8] destroy_context+0x58/0x340
  LR [c00000000013da18] __mmdrop+0x78/0x270
  Call Trace:
  [c000000f7db77c80] [c00000000013da18] __mmdrop+0x78/0x270
  [c000000f7db77cf0] [c0000000004d6a34] __do_execve_file.isra.13+0xbd4/0x1000
  [c000000f7db77e00] [c0000000004d7428] sys_execve+0x58/0x70
  [c000000f7db77e30] [c00000000000b388] system_call+0x5c/0x70

Reported-by: Priya M.A <priyama2@in.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
[mpe: Reformat/tweak comment wording]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190918140103.24395-1-aneesh.kumar@linux.ibm.com