powerpc/rtas: Keep MSR[RI] set when calling RTAS

In the Linux kernel, the following vulnerability has been resolved:

powerpc/rtas: Keep MSR[RI] set when calling RTAS

RTAS runs in real mode (MSR[DR] and MSR[IR] unset) and in 32-bit big
endian mode (MSR[SF,LE] unset).

The change in MSR is done in enter_rtas() in a relatively complex way,
since the MSR value could be hardcoded.

Furthermore, a panic has been reported when hitting the watchdog interrupt
while running in RTAS, this leads to the following stack trace:

watchdog: CPU 24 Hard LOCKUP
watchdog: CPU 24 TB:997512652051031, last heartbeat TB:997504470175378 (15980ms ago)
...
Supported: No, Unreleased kernel
CPU: 24 PID: 87504 Comm: drmgr Kdump: loaded Tainted: G E X 5.14.21-150400.71.1.bz196362_2-default #1 SLE15-SP4 (unreleased) 0d821077ef4faa8dfaf370efb5fdca1fa35f4e2c
NIP: 000000001fb41050 LR: 000000001fb4104c CTR: 0000000000000000
REGS: c00000000fc33d60 TRAP: 0100 Tainted: G E X (5.14.21-150400.71.1.bz196362_2-default)
MSR: 8000000002981000 <SF,VEC,VSX,ME> CR: 48800002 XER: 20040020
CFAR: 000000000000011c IRQMASK: 1
GPR00: 0000000000000003 ffffffffffffffff 0000000000000001 00000000000050dc
GPR04: 000000001ffb6100 0000000000000020 0000000000000001 000000001fb09010
GPR08: 0000000020000000 0000000000000000 0000000000000000 0000000000000000
GPR12: 80040000072a40a8 c00000000ff8b680 0000000000000007 0000000000000034
GPR16: 000000001fbf6e94 000000001fbf6d84 000000001fbd1db0 000000001fb3f008
GPR20: 000000001fb41018 ffffffffffffffff 000000000000017f fffffffffffff68f
GPR24: 000000001fb18fe8 000000001fb3e000 000000001fb1adc0 000000001fb1cf40
GPR28: 000000001fb26000 000000001fb460f0 000000001fb17f18 000000001fb17000
NIP [000000001fb41050] 0x1fb41050
LR [000000001fb4104c] 0x1fb4104c
Call Trace:
Instruction dump:
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
Oops: Unrecoverable System Reset, sig: 6 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
...
Supported: No, Unreleased kernel
CPU: 24 PID: 87504 Comm: drmgr Kdump: loaded Tainted: G E X 5.14.21-150400.71.1.bz196362_2-default #1 SLE15-SP4 (unreleased) 0d821077ef4faa8dfaf370efb5fdca1fa35f4e2c
NIP: 000000001fb41050 LR: 000000001fb4104c CTR: 0000000000000000
REGS: c00000000fc33d60 TRAP: 0100 Tainted: G E X (5.14.21-150400.71.1.bz196362_2-default)
MSR: 8000000002981000 <SF,VEC,VSX,ME> CR: 48800002 XER: 20040020
CFAR: 000000000000011c IRQMASK: 1
GPR00: 0000000000000003 ffffffffffffffff 0000000000000001 00000000000050dc
GPR04: 000000001ffb6100 0000000000000020 0000000000000001 000000001fb09010
GPR08: 0000000020000000 0000000000000000 0000000000000000 0000000000000000
GPR12: 80040000072a40a8 c00000000ff8b680 0000000000000007 0000000000000034
GPR16: 000000001fbf6e94 000000001fbf6d84 000000001fbd1db0 000000001fb3f008
GPR20: 000000001fb41018 ffffffffffffffff 000000000000017f fffffffffffff68f
GPR24: 000000001fb18fe8 000000001fb3e000 000000001fb1adc0 000000001fb1cf40
GPR28: 000000001fb26000 000000001fb460f0 000000001fb17f18 000000001fb17000
NIP [000000001fb41050] 0x1fb41050
LR [000000001fb4104c] 0x1fb4104c
Call Trace:
Instruction dump:
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX
---[ end trace 3ddec07f638c34a2 ]---

This happens because MSR[RI] is unset when entering RTAS but there is no
valid reason to not set it here.

RTAS is expected to be called with MSR[RI] as specified in PAPR+ section
"7.2.1 Machine State":

R1–7.2.1–9. If called with MSR[RI] equal to 1, then RTAS must protect
its own critical regions from recursion by setting the MSR[RI] bit to
0 when in the critical regions.

Fixing this by reviewing the way MSR is compute before calling RTAS. Now a
hardcoded value meaning real
---truncated---