XFS: Switch to native endian internal representation for extents

XFS: Switch to native endian internal representation for extents

Switch xfs from using a big endian internal representation for
the in memory copy of extents to a host byte order representation.
The internal extents are read in once, then modified seperately
from the on disk ones. Since we search and manipulate the extents
multiple times, it is cheaper to convert them to host byte order
once and then keep them in that format. Worth about 5 to 10%
reduction in cpu time for some loads. Complicated by the fact
that the in memory extents are written out to the log sometimes,
and when expanding extents are used to write out the initial
block of extents.

Modid: 2.5.x-xfs:slinx:129646a