mm/mincore.c: use vma_lookup() instead of find_vma()

Using vma_lookup() verifies the start address is contained in the found
vma.  This results in easier to read the code.

Link: https://lkml.kernel.org/r/20221007030345.5029-1-wangdeming@inspur.com


Signed-off-by: Deming Wang <wangdeming@inspur.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>