afs_TruncateAllSegments() make sure correct afs_size_t comparison is

afs_TruncateAllSegments() make sure correct afs_size_t comparison is
used when selecting dcache entries for truncation

Insure that afs_TruncateAllSegments() uses the correct [64-bit]
comparison when deciding whether to truncate chunks:
alen - AFS_CHUNKTOBASE(tdc->f.chunk) for a file > 2GB would fail
to be recognised negative with an "afs_int32 newSize", even with
alen = 0 and big tdc->f.chunk.

Change-Id: Id77109fb8238234dae725bd876f70ea0fbd403fc
Change-Id: If0120e21a52316e536d03241c83e8f0d36614c13
Reviewed-on: http://gerrit.openafs.org/1252
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>