powerpc: Fix handling of DSCR related facility unavailable exception

powerpc: Fix handling of DSCR related facility unavailable exception

Currently DSCR (Data Stream Control Register) can be accessed with
mfspr or mtspr instructions inside a thread via two different SPR
numbers. One being the user accessible problem state SPR number 0x03
and the other being the privilege state SPR number 0x11. All access
through the privilege state SPR number get emulated through illegal
instruction exception. Any access through the problem state SPR number
raises one facility unavailable exception which sets the thread based
dscr_inherit bit and enables DSCR facility through FSCR register thus
allowing direct access to DSCR without going through this exception in
the future. We set the thread.dscr_inherit bit whether the access was
with mfspr or mtspr instruction which is neither correct nor does it
match the behaviour through the instruction emulation code path driven
from privilege state SPR number. User currently observes two different
kind of behaviour when accessing the DSCR through these two SPR numbers.
This problem can be observed through these two test cases by replacing
the privilege state SPR number with the problem state SPR number.

	(1) http://ozlabs.org/~anton/junkcode/dscr_default_test.c
	(2) http://ozlabs.org/~anton/junkcode/dscr_explicit_test.c

This patch fixes the problem by making sure that the behaviour visible
to the user remains the same irrespective of which SPR number is being
used. Inside facility unavailable exception, we check whether it was
cuased by a mfspr or a mtspr isntrucction. In case of mfspr instruction,
just emulate the instruction. In case of mtspr instruction, set the
thread based dscr_inherit bit and also enable the facility through FSCR.
All user SPR based mfspr instruction will be emulated till one user SPR
based mtspr has been executed.

Signed-off-by: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>