ovl: rename is_merge to is_lowest

ovl: rename is_merge to is_lowest

commit 56656e960b555cb98bc414382566dcb59aae99a2 upstream.

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>
Signed-off-by: SZ Lin (林上智) <sz.lin@moxa.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[sw0312.kim: cherry-pick linux-4.4.y commit 6586f61ab5bb to fix smack deny issue on overlayfs]
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Change-Id: I00655e4e8286d2d0e16b61e6d11e559c60ec4036