ext4: store checksum seed in superblock

ext4: store checksum seed in superblock

Allow the filesystem to store the metadata checksum seed in the
superblock and add an incompat feature to say that we're using it.
This enables tune2fs to change the UUID on a mounted metadata_csum
FS without having to (racy!) rewrite all disk metadata.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
(cherry picked from commit 8c81bd8f586c46eaf114758a78d82895a2b081c2)

Orabug: 25802481
CVE-2016-10208

Signed-off-by: Kirtikar Kashyap <kirtikar.kashyap@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
 Conflicts:
	fs/ext4/sysfs.c