arm64: entry: Avoid empty alternatives entries

kernel_ventry will create alternative entries to potentially replace
0 instructions with 0 instructions for EL1 vectors. While this does not
cause an issue, it pointlessly takes up some bytes in the alternatives
section.

Do not generate such entries.
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NJulien Thierry <jthierry@redhat.com>
Signed-off-by: NWill Deacon <will@kernel.org>

arch/arm64/kernel/entry.S

@@ -60,16 +60,16 @@
 	.macro kernel_ventry, el, label, regsize = 64
 	.align 7
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
-alternative_if ARM64_UNMAP_KERNEL_AT_EL0
 	.if	\el == 0
+alternative_if ARM64_UNMAP_KERNEL_AT_EL0
 	.if	\regsize == 64
 	mrs	x30, tpidrro_el0
 	msr	tpidrro_el0, xzr
 	.else
 	mov	x30, xzr
 	.endif
-	.endif
 alternative_else_nop_endif
+	.endif
 #endif
 
 	sub	sp, sp, #S_FRAME_SIZE