mm/mremap: remove goto from mremap_to()

mremap_to() has a goto label at the end that doesn't unwind anything. 
Removing the label makes the code cleaner.

This commit also adds documentation to the function.

Link: https://lkml.kernel.org/r/20241018174114.2871880-3-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
Reviewed-by: Pedro Falcato <pedro.falcato@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Jeff Xu <jeffxu@chromium.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>