arm64: entry.S: Add ventry overflow sanity checks

Subsequent patches add even more code to the ventry slots.
Ensure kernels that overflow a ventry slot don't get built.
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>

arch/arm64/kernel/entry.S

@@ -37,6 +37,7 @@
 
 	.macro kernel_ventry, el:req, ht:req, regsize:req, label:req
 	.align 7
+.Lventry_start\@:
 #ifdef CONFIG_UNMAP_KERNEL_AT_EL0
 	.if	\el == 0
 alternative_if ARM64_UNMAP_KERNEL_AT_EL0
@@ -95,6 +96,7 @@ alternative_else_nop_endif
 	mrs	x0, tpidrro_el0
 #endif
 	b	el\el\ht\()_\regsize\()_\label
+.org .Lventry_start\@ + 128	// Did we overflow the ventry slot?
 	.endm
 
 	.macro tramp_alias, dst, sym
@@ -662,6 +664,7 @@ alternative_else_nop_endif
 	add	x30, x30, #(1b - tramp_vectors)
 	isb
 	ret
+.org 1b + 128	// Did we overflow the ventry slot?
 	.endm
 
 	.macro tramp_exit, regsize = 64