mm/memory.c: fix potential pte_unmap_unlock pte error

mm/memory.c: fix potential pte_unmap_unlock pte error

If all pte entry is none in 'non-create' case, we would break the loop with
pte unchanged.  Then the wrong pte - 1 would be passed to pte_unmap_unlock.
This is a theoretical issue which may not be a real bug. So it's not worth
cc stable.

Link: https://lkml.kernel.org/r/20210205081925.59809-1-linmiaohe@huawei.com
Fixes: cc08b15ed67d ("Add apply_to_page_range() which applies a function to a pte range")
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Ian Pratt <ian.pratt@xensource.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>