arm64: spinlock: fix ll/sc unlock on big-endian systems

When unlocking a spinlock, we perform a read-modify-write on the owner
ticket in order to increment it and store it back with release
semantics.

In the LL/SC case, we load the 16-bit ticket using a 32-bit load and
therefore store back the wrong halfword on a big-endian system,
corrupting the lock after the first unlock and killing the system dead.

This patch fixes the unlock code to use 16-bit accessors consistently.
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/include/asm/spinlock.h

@@ -110,7 +110,7 @@ static inline void arch_spin_unlock(arch_spinlock_t *lock)
 
 	asm volatile(ARM64_LSE_ATOMIC_INSN(
 	/* LL/SC */
-	"	ldr	%w1, %0\n"
+	"	ldrh	%w1, %0\n"
 	"	add	%w1, %w1, #1\n"
 	"	stlrh	%w1, %0",
 	/* LSE atomics */