arm64: simplify contextidr_thread_switch

When CONFIG_PID_IN_CONTEXTIDR is not selected, we use an empty stub
definition of contextidr_thread_switch(). As everything we rely upon
exists regardless of CONFIG_PID_IN_CONTEXTIDR, we don't strictly require
an empty stub.

By using IS_ENABLED() rather than ifdeffery, we avoid duplication, and
get compiler coverage on all the code even when CONFIG_PID_IN_CONTEXTIDR
is not selected and the code is optimised away.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/mmu_context.h

@@ -30,17 +30,14 @@
 #include <asm/sysreg.h>
 #include <asm/tlbflush.h>
 
-#ifdef CONFIG_PID_IN_CONTEXTIDR
 static inline void contextidr_thread_switch(struct task_struct *next)
 {
+	if (!IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR))
+		return;
+
 	write_sysreg(task_pid_nr(next), contextidr_el1);
 	isb();
 }
-#else
-static inline void contextidr_thread_switch(struct task_struct *next)
-{
-}
-#endif
 
 /*
  * Set TTBR0 to empty_zero_page. No translations will be possible via TTBR0.