arm64/alternatives: move length validation inside the subsection

[ Upstream commit 966a0acce2fca776391823381dba95c40e03c339 ]

Commit f7b93d42945c ("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: f7b93d42945c ("arm64/alternatives: use subsections for replacement sequences")
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Link: ClangBuiltLinux/linux#1078
Link: https://lore.kernel.org/r/20200730153701.3892953-1-samitolvanen@google.com
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/arm64/include/asm/alternative.h

@@ -74,7 +74,8 @@ void apply_alternatives(void *start, size_t length);
 	"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)	\