Use an inline pxd list rather than an xad list in the xadlock.
When the number of extents being modified can fit with the xadlock,
a transaction can be committed asynchronously.  Using a list of
pxd's instead of xad's allows us to fit 4 extents, rather than 2.
Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>