brd: re-enable __GFP_HIGHMEM in brd_insert_page()

__GFP_HIGHMEM is disabled if dax is enabled on brd, however
dax support for brd has been removed since commit (7a862fbb
"brd: remove dax support"), so restore __GFP_HIGHMEM in
brd_insert_page().

Also remove the no longer applicable comments about DAX and highmem.

Cc: stable@vger.kernel.org
Fixes: 7a862fbb ("brd: remove dax support")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/brd.c

@@ -96,13 +96,8 @@ static struct page *brd_insert_page(struct brd_device *brd, sector_t sector)
 	/*
 	 * Must use NOIO because we don't want to recurse back into the
 	 * block or filesystem layers from page reclaim.
-	 *
-	 * Cannot support DAX and highmem, because our ->direct_access
-	 * routine for DAX must return memory that is always addressable.
-	 * If DAX was reworked to use pfns and kmap throughout, this
-	 * restriction might be able to be lifted.
 	 */
-	gfp_flags = GFP_NOIO | __GFP_ZERO;
+	gfp_flags = GFP_NOIO | __GFP_ZERO | __GFP_HIGHMEM;
 	page = alloc_page(gfp_flags);
 	if (!page)
 		return NULL;