[XFS] Clean up i_flags and i_flags_lock handling.

SGI-PV: 956832
SGI-Modid: xfs-linux-melb:xfs-kern:27358a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nscott@aconex.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>