ovl: move super block magic number to magic.h

ovl: move super block magic number to magic.h

Orabug: 26546379, 26540706
CVE-2016-1575
CVE-2016-1576

The overlayfs file system is not recognized by programs
like tail because the magic number is not in standard header location.

Move it so that the value will propagate on for the GNU library
and utilities. Needs to go in the fstatfs manual page as well.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
(cherry picked from commit 257f871993474e2bde6c497b54022c362cf398e1)
Signed-off-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>