s390: fix kernel asce loading when sie is interrupted

If a machine check is coming in during sie, the PU saves the
control registers to the machine check save area. Afterwards
mcck_int_handler is called, which loads __LC_KERNEL_ASCE into
%cr1. Later the code restores %cr1 from the machine check area,
but that is wrong when SIE was interrupted because the machine
check area still contains the gmap asce. Instead it should return
with either __KERNEL_ASCE in %cr1 when interrupted in SIE or
the previous %cr1 content saved in the machine check save area.

Fixes: 87d59863 ("s390/mm: remove set_fs / rework address space handling")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Cc: <stable@kernel.org> # v5.8+
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/kernel/entry.S

@@ -558,6 +558,7 @@ ENTRY(mcck_int_handler)
 	lg	%r15,__LC_MCCK_STACK
 .Lmcck_skip:
 	la	%r11,STACK_FRAME_OVERHEAD(%r15)
+	stctg	%c1,%c1,__PT_CR1(%r11)
 	lctlg	%c1,%c1,__LC_KERNEL_ASCE
 	xc	__SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
 	lghi	%r14,__LC_GPREGS_SAVE_AREA+64
@@ -573,8 +574,6 @@ ENTRY(mcck_int_handler)
 	xgr	%r10,%r10
 	mvc	__PT_R8(64,%r11),0(%r14)
 	stmg	%r8,%r9,__PT_PSW(%r11)
-	la	%r14,4095
-	mvc	__PT_CR1(8,%r11),__LC_CREGS_SAVE_AREA-4095+8(%r14)
 	xc	__PT_FLAGS(8,%r11),__PT_FLAGS(%r11)
 	xc	__SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
 	lgr	%r2,%r11		# pass pointer to pt_regs