Per process DSCR + somefixes

This is reworked http://patchwork.ozlabs.org/patch/71489/

Originally there were 4 questions left:

1. /sys/devices/system/cpu/cpu##/dscr
It is left as is.

2. Out of band method of changing the DSCR:
Introduced new /sys/devices/system/cpu/dscr_default and "dscr_inherit" flag for a thread's DSCR which is used in copy_thread to decide whether to inherit a DSCR value or not.
The "inherit" flag is set when:
- DSCR has been explicitly changed by writing DSCR;
- a parent process has this flag set;
- dscr_default is not 0 at the moment of fork().
In all other cases DSCR remains zero.

3. improve context switch speed.
Done :-)

4. About disabling the feature when there is "no stinking DSCR".
The calls likes:
cpu_has_feature(CPU_FTR_DSCR)
and
BEGIN_FTR_SECTION
END_FTR_SECTION_IFSET(CPU_FTR_DSCR)
ensure that DSCR is not accessed on systems which do not support it.

Tested on power5, power6, power7.


-- 
Alexey Kardashevskiy
IBM OzLabs, LTC Team

e-mail/sametime: aik@au1.ibm.com
notes: Alexey Kardashevskiy/Australia/IBM