userfaultfd: fix remap event with MREMAP_DONTUNMAP

A user is not required to set a new address when using MREMAP_DONTUNMAP
as it can be used without MREMAP_FIXED.  When doing so the remap event
will use new_addr which may not have been set and we didn't propagate it
back other then in the return value of remap_to.

Because ret is always the new address it's probably more correct to use
it rather than new_addr on the remap_event_complete call, and it
resolves this bug.

Fixes: e346b381 ("mm/mremap: add MREMAP_DONTUNMAP to mremap()")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Brian Geffon <bgeffon@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: "Michael S . Tsirkin" <mst@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Sonny Rao <sonnyrao@google.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Link: http://lkml.kernel.org/r/20200506172158.218366-1-bgeffon@google.com


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>