bpf: crypto: replace -EEXIST with -EBUSY

The -EEXIST error code is reserved by the module loading infrastructure
to indicate that a module is already loaded. When a module's init
function returns -EEXIST, userspace tools like kmod interpret this as
"module already loaded" and treat the operation as successful, returning
0 to the user even though the module initialization actually failed.

This follows the precedent set by commit 54416fd ("netfilter:
conntrack: helper: Replace -EEXIST by -EBUSY") which fixed the same
issue in nf_conntrack_helper_register().

This affects bpf_crypto_skcipher module. While the configuration
required to build it as a module is unlikely in practice, it is
technically possible, so fix it for correctness.

Signed-off-by: Daniel Gomez <da.gomez@samsung.com>

Author: dkruces

kernel/bpf/crypto.c

@@ -60,7 +60,7 @@ struct bpf_crypto_ctx {
 int bpf_crypto_register_type(const struct bpf_crypto_type *type)
 {
 	struct bpf_crypto_type_list *node;
-	int err = -EEXIST;
+	int err = -EBUSY;
 
 	down_write(&bpf_crypto_types_sem);
 	list_for_each_entry(node, &bpf_crypto_types, list) {