Refactor the fallocate call.

@sueloverso, here's a (very lightly tested) branch to refactor the fallocate call.
Even though NOT_AVAILABLE is 0 here, it would be cleaner if the block and log code checking that field explicitly checked for` WT_FALLOCATE_NOT_AVAILABLE` now rather than assume it is 0.  Then this list could be alphabetized too.