mm/mremap: avoid unneeded do_munmap call

When old_len == new_len, do_munmap will return -EINVAL due to len == 0. 
This errno will be simply ignored because of old_len != new_len check.  So
it is unnecessary to call do_munmap when old_len == new_len because
nothing is actually done.

Link: https://lkml.kernel.org/r/20220401081023.37080-1-linmiaohe@huawei.com


Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>