mm/migrate.c: remove unnecessary rc != MIGRATEPAGE_SUCCESS check in 'else' case

mm/migrate.c: remove unnecessary rc != MIGRATEPAGE_SUCCESS check in 'else' case

It's guaranteed that in the 'else' case of the rc == MIGRATEPAGE_SUCCESS
check, rc does not equal to MIGRATEPAGE_SUCCESS.  Remove this unnecessary
check.

Link: https://lkml.kernel.org/r/20210325131524.48181-3-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Jerome Glisse <jglisse@redhat.com>
Cc: Rafael Aquini <aquini@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>