Unix CM: Fix negative file length case, again

Unix CM: Fix negative file length case, again

As originally noted in f6f9ee5402f1718f330a00ec89fb34b05c3cd360
some fileservers return a negative length, typically when a client
is attempting to fetch data that is past the extents of the file,
the CM needs to retain this negative length, and handle it correctly.

c7b92a3018044f7aca4d9a77644e5c06ef64d1e9 added safety checks for
the fileserver returning a length larger than that asked for by
the client. Sadly, this check does a comparison between a
signed, and an unsigned, variable. This leads to it incorrectly
classifying negative responses as being too large.

Add a cast, and a comment, to fix this.

Change-Id: I2ca67f55204c565667d5cd91cde3d520f8d9b10c
Reviewed-on: http://gerrit.openafs.org/1109
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>