sparc64: fix fault handling in NGbzero.S and GENbzero.S

When any of the functions contained in NGbzero.S and GENbzero.S
vector through *bzero_from_clear_user, we may end up taking a
fault when executing one of the store alternate address space
instructions. If this happens, the exception handler does not
restore the %asi register.

This commit fixes the issue by introducing a new exception
handler that ensures the %asi register is restored when
a fault is handled.

Orabug: 25577560
Signed-off-by: NDave Aldridge <david.j.aldridge@oracle.com>
Reviewed-by: NRob Gardner <rob.gardner@oracle.com>
Reviewed-by: NBabu Moger <babu.moger@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/kernel/head_64.S

@@ -939,3 +939,9 @@ ENTRY(__retl_o1)
 	retl
 	 mov	%o1, %o0
 ENDPROC(__retl_o1)
+
+ENTRY(__retl_o1_asi)
+	wr      %o5, 0x0, %asi
+	retl
+	 mov    %o1, %o0
+ENDPROC(__retl_o1_asi)

arch/sparc/lib/GENbzero.S

@@ -8,7 +8,7 @@
 98:	x,y;			\
 	.section __ex_table,"a";\
 	.align 4;		\
-	.word 98b, __retl_o1;	\
+	.word 98b, __retl_o1_asi;\
 	.text;			\
 	.align 4;
 

arch/sparc/lib/NGbzero.S

@@ -8,7 +8,7 @@
 98:	x,y;			\
 	.section __ex_table,"a";\
 	.align 4;		\
-	.word 98b, __retl_o1;	\
+	.word 98b, __retl_o1_asi;\
 	.text;			\
 	.align 4;