mm: gup: remove useless BUG_ON in __get_user_pages()

mm: gup: remove useless BUG_ON in __get_user_pages()

Indeed, this BUG_ON couldn't catch anything useful.  We are sure ret == 0
here because we would already bail out if ret != 0 and ret is untouched
till here.

Link: https://lkml.kernel.org/r/20210807093620.21347-4-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>