ovl: rename is_merge to is_lowest

ovl: rename is_merge to is_lowest

The 'is_merge' is an historical naming from when only a single lower layer
could exist.  With the introduction of multiple lower layers the meaning of
this flag was changed to mean only the "lowest layer" (while all lower
layers were being merged).

So now 'is_merge' is inaccurate and hence renaming to 'is_lowest'

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Orabug: 26401569

(backport upstream commit 56656e960b555cb98bc414382566dcb59aae99a2)

Signed-off-by: Shan Hai <shan.hai@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>