kthread: simplify kthread_use_mm refcounting

kthread: simplify kthread_use_mm refcounting

Patch series "shoot lazy tlbs (lazy tlb refcount scalability
improvement)", v7.

This series improves scalability of context switching between user and
kernel threads on large systems with a threaded process spread across a
lot of CPUs.

Discussion of v6 here:
https://lore.kernel.org/linux-mm/20230118080011.2258375-1-npiggin@gmail.com/

This patch (of 5):

Remove the special case avoiding refcounting when the mm to be used is the
same as the kernel thread's active (lazy tlb) mm.  kthread_use_mm() should
not be such a performance critical path that this matters much.  This
simplifies a later change to lazy tlb mm refcounting.

Link: https://lkml.kernel.org/r/20230203071837.1136453-1-npiggin@gmail.com
Link: https://lkml.kernel.org/r/20230203071837.1136453-2-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>