The ExpressMIMO2 kernel driver did not use proper mechanisms
to copy from user space.

It is very surprising that it ever worked at all...

This commit is a quick fix. It is not the end of the story
and some more work may be needed if things don't work.

The remaining issue is that we pass pointer addresses to the
kernel as 32 bits values, but pointers are 64 bits values
(on x86_64, our supported platform). Some checks have been
put in place to detect if upper 32 bits of a pointer are
not 0 and print a strong error message in case of problem,
but no clean solution has been implemented.

usrp 20 MHz on 30.72 master clock. fixed memory leak in prach.c for 20 MHz with 3/4 sampling. added default attributes to threads in lte-enb.c