random: avoid warnings for !CONFIG_NUMA builds

As crng_initialize_secondary() is only called by do_numa_crng_init(),
and the latter is under ifdeffery for CONFIG_NUMA, when CONFIG_NUMA is
not selected the compiler will warn that the former is unused:

| drivers/char/random.c:820:13: warning: 'crng_initialize_secondary' defined but not used [-Wunused-function]
|   820 | static void crng_initialize_secondary(struct crng_state *crng)
|       |             ^~~~~~~~~~~~~~~~~~~~~~~~~

Stephen reports that this happens for x86_64 noallconfig builds.

We could move crng_initialize_secondary() and crng_init_try_arch() under
the CONFIG_NUMA ifdeffery, but this has the unfortunate property of
separating them from crng_initialize_primary() and
crng_init_try_arch_early() respectively. Instead, let's mark
crng_initialize_secondary() as __maybe_unused.

Link: https://lore.kernel.org/r/20200310121747.GA49602@lakrids.cambridge.arm.com
Fixes: 5cbe0f13 ("random: split primary/secondary crng init paths")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

drivers/char/random.c

@@ -817,7 +817,7 @@ static bool __init crng_init_try_arch_early(struct crng_state *crng)
 	return arch_init;
 }
 
-static void crng_initialize_secondary(struct crng_state *crng)
+static void __maybe_unused crng_initialize_secondary(struct crng_state *crng)
 {
 	memcpy(&crng->state[0], "expand 32-byte k", 16);
 	_get_random_bytes(&crng->state[4], sizeof(__u32) * 12);