selftests/mm: gup_test: fix comment regarding origin of FOLL_WRITE

selftests/mm: gup_test: fix comment regarding origin of FOLL_WRITE

The 'FOLL_WRITE' of the copied source is located in mm_types.h of mm, not
mm.h, so fix it.

Link: https://lkml.kernel.org/r/20251117154012.197499-2-peng8420.li@gmail.com
Signed-off-by: Peng Li <peng8420.li@gmail.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>