s390/entry: Scrub r12 register on kernel entry

Before commit f33f2d4 ("s390/bp: remove TIF_ISOLATE_BP"),
all entry handlers loaded r12 with the current task pointer
(lg %r12,__LC_CURRENT) for use by the BPENTER/BPEXIT macros. That
commit removed TIF_ISOLATE_BP, dropping both the branch prediction
macros and the r12 load, but did not add r12 to the register clearing
sequence.

Add the missing xgr %r12,%r12 to make the register scrub consistent
across all entry points.

Fixes: f33f2d4 ("s390/bp: remove TIF_ISOLATE_BP")
Cc: [email protected]
Reviewed-by: Ilya Leoshkevich <[email protected]>
Signed-off-by: Vasily Gorbik <[email protected]>

Author: Vasily Gorbik

arch/s390/kernel/entry.S

@@ -271,6 +271,7 @@ SYM_CODE_START(system_call)
 	xgr	%r9,%r9
 	xgr	%r10,%r10
 	xgr	%r11,%r11
+	xgr	%r12,%r12
 	la	%r2,STACK_FRAME_OVERHEAD(%r15)	# pointer to pt_regs
 	mvc	__PT_R8(64,%r2),__LC_SAVE_AREA(%r13)
 	MBEAR	%r2,%r13
@@ -407,6 +408,7 @@ SYM_CODE_START(\name)
 	xgr	%r6,%r6
 	xgr	%r7,%r7
 	xgr	%r10,%r10
+	xgr	%r12,%r12
 	xc	__PT_FLAGS(8,%r11),__PT_FLAGS(%r11)
 	mvc	__PT_R8(64,%r11),__LC_SAVE_AREA(%r13)
 	MBEAR	%r11,%r13
@@ -496,6 +498,7 @@ SYM_CODE_START(mcck_int_handler)
 	xgr	%r6,%r6
 	xgr	%r7,%r7
 	xgr	%r10,%r10
+	xgr	%r12,%r12
 	stmg	%r8,%r9,__PT_PSW(%r11)
 	xc	__PT_FLAGS(8,%r11),__PT_FLAGS(%r11)
 	xc	__SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)