st.ko: some changes for enlarge_buffer

Hi Kai,

here is a short series of 3 small patches for st.ko

The first one removes a bug in enlarge_buffer() that can make a read or write fail under special conditions.

The second one removes a small piece of code in enlarge_buffer(), that AFAICS is not necessary.

If enlarge_buffer() fails, the errno currently presented to userland always is EOVERFLOW, while I think it sometimes should be ENOMEM. The third patch is a try to enhance this.

All three patches are slightly tested under SLES11-SP2 kernel
3.0.80-0.7.1-xxxxx and after this adapted to 3.12.2 (just had to fix
some offset). They compile fine under 3.12.2

HTH

Thanks,
Bodo