arm64/alternatives: move length validation inside the subsection

Commit f7b93d4 ("arm64/alternatives: use subsections for replacement
sequences") breaks LLVM's integrated assembler, because due to its
one-pass design, it cannot compute instruction sequence lengths before the
layout for the subsection has been finalized. This change fixes the build
by moving the .org directives inside the subsection, so they are processed
after the subsection layout is known.

Fixes: f7b93d4 ("arm64/alternatives: use subsections for replacement sequences")
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Change-Id: If386e0c33faab585b7213653ae2c7c910cc479f5
Link: ClangBuiltLinux/linux#1078
Link: https://lore.kernel.org/r/20200730153701.3892953-1-samitolvanen@google.com
Signed-off-by: TogoFire <togofire@mailfence.com>

arch/arm64/include/asm/alternative.h

@@ -71,9 +71,9 @@ void apply_alternatives(void *start, size_t length);
 	"663:\n\t"							\
 	newinstr "\n"							\
 	"664:\n\t"							\
-	".popsection\n\t"						\
 	".org	. - (664b-663b) + (662b-661b)\n\t"			\
-	".org	. - (662b-661b) + (664b-663b)\n"			\
+	".org	. - (662b-661b) + (664b-663b)\n\t"			\
+	".previous\n"							\
 	".endif\n"
 
 #define __ALTERNATIVE_CFG_CB(oldinstr, feature, cfg_enabled, cb)	\