powerpc/8xx: Reduce time spent in allow_user_access() and friends

To enable/disable kernel access to user space, the 8xx has to
modify the properties of access group 1. This is done by writing
predefined values into SPRN_Mx_AP registers.

As of today, a __put_user() gives:

00000d64 <my_test>:
 d64:	3d 20 4f ff 	lis     r9,20479
 d68:	61 29 ff ff 	ori     r9,r9,65535
 d6c:	7d 3a c3 a6 	mtspr   794,r9
 d70:	39 20 00 00 	li      r9,0
 d74:	90 83 00 00 	stw     r4,0(r3)
 d78:	3d 20 6f ff 	lis     r9,28671
 d7c:	61 29 ff ff 	ori     r9,r9,65535
 d80:	7d 3a c3 a6 	mtspr   794,r9
 d84:	4e 80 00 20 	blr

Because only groups 0 and 1 are used, the definition of
groups 2 to 15 doesn't matter.
By setting unused bits to 0 instead on 1, one instruction is
removed for each lock and unlock action:

00000d5c <my_test>:
 d5c:	3d 20 40 00 	lis     r9,16384
 d60:	7d 3a c3 a6 	mtspr   794,r9
 d64:	39 20 00 00 	li      r9,0
 d68:	90 83 00 00 	stw     r4,0(r3)
 d6c:	3d 20 60 00 	lis     r9,24576
 d70:	7d 3a c3 a6 	mtspr   794,r9
 d74:	4e 80 00 20 	blr
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/57425c33dd72f292b1a23570244b81419072a7aa.1586945153.git.christophe.leroy@c-s.fr

arch/powerpc/include/asm/nohash/32/mmu-8xx.h

@@ -36,16 +36,16 @@
  * Therefore, we define 2 APG groups. lsb is _PMD_USER
  * 0 => Kernel => 01 (all accesses performed according to page definition)
  * 1 => User => 00 (all accesses performed as supervisor iaw page definition)
- * 2-16 => NA => 11 (all accesses performed as user iaw page definition)
+ * 2-15 => Not Used
  */
-#define MI_APG_INIT	0x4fffffff
+#define MI_APG_INIT	0x40000000
 
 /*
  * 0 => Kernel => 01 (all accesses performed according to page definition)
  * 1 => User => 10 (all accesses performed according to swaped page definition)
- * 2-16 => NA => 11 (all accesses performed as user iaw page definition)
+ * 2-15 => Not Used
  */
-#define MI_APG_KUEP	0x6fffffff
+#define MI_APG_KUEP	0x60000000
 
 /* The effective page number register.  When read, contains the information
  * about the last instruction TLB miss.  When MI_RPN is written, bits in
@@ -115,16 +115,16 @@
  * Therefore, we define 2 APG groups. lsb is _PMD_USER
  * 0 => Kernel => 01 (all accesses performed according to page definition)
  * 1 => User => 00 (all accesses performed as supervisor iaw page definition)
- * 2-16 => NA => 11 (all accesses performed as user iaw page definition)
+ * 2-15 => Not Used
  */
-#define MD_APG_INIT	0x4fffffff
+#define MD_APG_INIT	0x40000000
 
 /*
  * 0 => No user => 01 (all accesses performed according to page definition)
  * 1 => User => 10 (all accesses performed according to swaped page definition)
- * 2-16 => NA => 11 (all accesses performed as user iaw page definition)
+ * 2-15 => Not Used
  */
-#define MD_APG_KUAP	0x6fffffff
+#define MD_APG_KUAP	0x60000000
 
 /* The effective page number register.  When read, contains the information
  * about the last instruction TLB miss.  When MD_RPN is written, bits in