powerpc/32s: Speed up likely path of kuap_update_sr()

In most cases, kuap_update_sr() will update a single segment
register.

We know that first update will always be done, if there is no
segment register to update at all, kuap_update_sr() is not
called.

Avoid recurring calculations and tests in that case.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/848f18d213b8341939add7302dc4ef80cc7a12e3.1620307636.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/book3s/32/kup.h

@@ -15,11 +15,13 @@ static inline void kuap_update_sr(u32 sr, u32 addr, u32 end)
 {
 	addr &= 0xf0000000;	/* align addr to start of segment */
 	barrier();	/* make sure thread.kuap is updated before playing with SRs */
-	while (addr < end) {
+	for (;;) {
 		mtsr(sr, addr);
+		addr += 0x10000000;	/* address of next segment */
+		if (addr >= end)
+			break;
 		sr += 0x111;		/* next VSID */
 		sr &= 0xf0ffffff;	/* clear VSID overflow */
-		addr += 0x10000000;	/* address of next segment */
 	}
 	isync();	/* Context sync required after mtsr() */
 }