Windows: cm_GetBuffer does not need to contact file server when extended a file

Windows: cm_GetBuffer does not need to contact file server when extended a file

If cm_GetBuffer is being called in order to obtain a buffer to store
data beyond the end of the existing file as known to the file server
there is no reason to contact the file server.  Instead use the cached
status info in order to allocate a new buffer zero initialized.

This logic avoids triggering the FetchData bug in all file servers
older than 1.4.12 and 1.5.65 in which the file server returns a
large negative number (filesize - requested_offset) when a FetchData
is received where the requested_offset is larger than the filesize.
It also avoids unnecessary work.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/542
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>