powerpc/64s/hash: Add stress_slb kernel boot option to increase SLB faults

This option increases the number of SLB misses by limiting the number
of kernel SLB entries, and increased flushing of cached lookaside
information. This helps stress test difficult to hit paths in the
kernel.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Relocate the code into arch/powerpc/mm, s/torture/stress/]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200511125825.3081305-1-mpe@ellerman.id.au