xfs: consistently initialize di_flags2

Make sure di_flags2 is always initialized.  We currently get this implicitly
by clearing the dinode core on allocating the in-core inode, but that is
about to go away.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>