lazy tlb: allow lazy tlb mm refcounting to be configurable

lazy tlb: allow lazy tlb mm refcounting to be configurable

Add CONFIG_MMU_TLB_REFCOUNT which enables refcounting of the lazy tlb mm
when it is context switched.  This can be disabled by architectures that
don't require this refcounting if they clean up lazy tlb mms when the last
refcount is dropped.  Currently this is always enabled, which is what
existing code does, so the patch is effectively a no-op.

Rename rq->prev_mm to rq->prev_lazy_mm, because that's what it is.

Link: https://lkml.kernel.org/r/20210605014216.446867-3-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Anton Blanchard <anton@ozlabs.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@ozlabs.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>