powerpc: Avoid link stack corruption in MMU on syscall entry path

Currently we use the link register to branch up high in the early MMU on
syscall entry path.  Unfortunately, this trashes the link stack as the
address we are going to is not associated with the earlier mflr.

This patch simply converts us to used the count register (volatile over
syscalls anyway) instead.  This is much better at predicting in this
scenario and doesn't trash link stack causing a bunch of additional
branch mispredicts later.  Benchmarking this on POWER8 saves a bunch of
cycles on Anton's null syscall benchmark here:
   http://ozlabs.org/~anton/junkcode/null_syscall.c

Signed-off-by: Michael Neuling <[email protected]>
Signed-off-by: Benjamin Herrenschmidt <[email protected]>

arch/powerpc/kernel/exceptions-64s.S

@@ -74,13 +74,13 @@ END_FTR_SECTION_IFSET(CPU_FTR_REAL_LE)				\
 	mflr	r10 ;						\
 	ld	r12,PACAKBASE(r13) ; 				\
 	LOAD_HANDLER(r12, system_call_entry_direct) ;		\
-	mtlr	r12 ;						\
+	mtctr	r12 ;						\
 	mfspr	r12,SPRN_SRR1 ;					\
 	/* Re-use of r13... No spare regs to do this */	\
 	li	r13,MSR_RI ;					\
 	mtmsrd 	r13,1 ;						\
 	GET_PACA(r13) ;	/* get r13 back */			\
-	blr ;
+	bctr ;
 #else
 	/* We can branch directly */
 #define SYSCALL_PSERIES_2_DIRECT				\