When using cp to copy a file under the following circumstanes:
- original file in on an NFS filesystem
- destination file is on the same NFS filesystem
- the file is less than 8Mbytes in size
- the file is larger than 65536 bytes in size
the cp process can get frozen in device-wait and never wake up (cp uses
mmap() in this case).
A small change to allocbuf() fixes this.