ovl: Add comment on upperredirect reassignment

If memory for uperredirect was allocated with kstrdup() in upperdir != NULL
and d.redirect != NULL path, it may seem that it can be lost when
upperredirect is reassigned later, but it's not possible.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 0a2d0d3f ("ovl: Check redirect on index as well")
Signed-off-by: Stanislav Goriainov <goriainov@ispras.ru>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>