mkfs: preserve DIFLAG2_NREXT64 when setting other inode attributes

Preserve the state of the NREXT64 inode flag when we're changing the
other flags2 fields.  This is only vital for the kernel version of this
function, but we should keep these in sync.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>