xfs: preserve DIFLAG2_NREXT64 when setting other inode attributes

It is vitally important that we preserve the state of the NREXT64 inode
flag when we're changing the other flags2 fields.

Fixes: 9b7d16e3

 ("xfs: Introduce XFS_DIFLAG2_NREXT64 and associated helpers")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>