Use min() to simplify the dmirror_exclusive() function and improve its
readability.

Link: https://lkml.kernel.org/r/20240726131245.161695-1-thorsten.blum@toblux.comSigned-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>