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Also remove the no longer applicable comments about DAX and highmem.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