[XFS] Explain the race closed by the addition of vn_iowait() to the start

of xfs_itruncate_start().

SGI-PV: 947420
SGI-Modid: xfs-linux-melb:xfs-kern:25527a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>