s390/kvm: fix detection of guest machine checks

The new detection code for guest machine checks added a check based
on %r11 to .Lcleanup_sie to distinguish between normal asynchronous
interrupts and machine checks. But the funtion is called from the
program check handler as well with an undefined value in %r11.

The effect is that all program exceptions pointing to the SIE instruction
will set the CIF_MCCK_GUEST bit. The bit stays set for the CPU until the
 next machine check comes in which will incorrectly be interpreted as a
guest machine check.

The simplest fix is to stop using .Lcleanup_sie in the program check
handler and duplicate a few instructions.

Fixes: c929500 ("s390/nmi: s390: New low level handling for machine check happening in guest")
Cc: <stable@vger.kernel.org> # v4.13+
Reviewed-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/entry.S

@@ -521,12 +521,15 @@ ENTRY(pgm_check_handler)
 	tmhh	%r8,0x0001		# test problem state bit
 	jnz	2f			# -> fault in user space
 #if IS_ENABLED(CONFIG_KVM)
-	# cleanup critical section for sie64a
+	# cleanup critical section for program checks in sie64a
 	lgr	%r14,%r9
 	slg	%r14,BASED(.Lsie_critical_start)
 	clg	%r14,BASED(.Lsie_critical_length)
 	jhe	0f
-	brasl	%r14,.Lcleanup_sie
+	lg	%r14,__SF_EMPTY(%r15)		# get control block pointer
+	ni	__SIE_PROG0C+3(%r14),0xfe	# no longer in SIE
+	lctlg	%c1,%c1,__LC_USER_ASCE		# load primary asce
+	larl	%r9,sie_exit			# skip forward to sie_exit
 #endif
 0:	tmhh	%r8,0x4000		# PER bit set in old PSW ?
 	jnz	1f			# -> enabled, can't be a double fault