random: do not export add_vmfork_randomness() unless needed

Since add_vmfork_randomness() is only called from vmgenid.o, we can
guard it in CONFIG_VMGENID, similarly to how we do with
add_disk_randomness() and CONFIG_BLOCK. If we ever have multiple things
calling into add_vmfork_randomness(), we can add another shared Kconfig
symbol for that, but for now, this is good enough. Even though
add_vmfork_randomess() is a pretty small function, removing it means
that there are only calls to crng_reseed(false) and none to
crng_reseed(true), which means the compiler can constant propagate the
false, removing branches from crng_reseed() and its descendants.

Additionally, we don't even need the symbol to be exported if
CONFIG_VMGENID is not a module, so conditionalize that too.

Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -1168,6 +1168,7 @@ void add_bootloader_randomness(const void *buf, size_t size)
 }
 EXPORT_SYMBOL_GPL(add_bootloader_randomness);
 
+#if IS_ENABLED(CONFIG_VMGENID)
 /*
  * Handle a new unique VM ID, which is unique, not secret, so we
  * don't credit it, but we do immediately force a reseed after so
@@ -1181,7 +1182,10 @@ void add_vmfork_randomness(const void *unique_vm_id, size_t size)
 		pr_notice("crng reseeded due to virtual machine fork\n");
 	}
 }
+#if IS_MODULE(CONFIG_VMGENID)
 EXPORT_SYMBOL_GPL(add_vmfork_randomness);
+#endif
+#endif
 
 struct fast_pool {
 	union {

include/linux/random.h

@@ -34,7 +34,9 @@ extern void add_input_randomness(unsigned int type, unsigned int code,
 extern void add_interrupt_randomness(int irq) __latent_entropy;
 extern void add_hwgenerator_randomness(const void *buffer, size_t count,
 				       size_t entropy);
+#if IS_ENABLED(CONFIG_VMGENID)
 extern void add_vmfork_randomness(const void *unique_vm_id, size_t size);
+#endif
 
 extern void get_random_bytes(void *buf, size_t nbytes);
 extern int wait_for_random_bytes(void);