[XFS] Avoid using inodes that haven't been completely initialised

The radix tree walks in xfs_sync_inodes_ag and xfs_qm_dqrele_all_inodes()
can find inodes that are still undergoing initialisation. Avoid
them by checking for the the XFS_INEW() flag once we have a reference
on the inode. This flag is cleared once the inode is properly initialised.

SGI-PV: 987246

Signed-off-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>