ovl: Annotate struct ovl_entry with __counted_by()

Add the __counted_by() compiler attribute to the flexible array member
'__lowerstack' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS
and CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>