[XFS] Implement the di_extsize allocator hint for non-realtime files as

well.  Also provides a mechanism for inheriting this property from the
parent directory for new files.

SGI-PV: 945264
SGI-Modid: xfs-linux-melb:xfs-kern:24367a

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

Author: natoscott