client: don't use uninitialized keyid in fscrypt_dummy_encryption

client: don't use uninitialized keyid in fscrypt_dummy_encryption

When add_fscrypt_key() failed it jumped to the err label, which then
read the uninitialized keyid buffer into a key specifier and issued a
key removal for it. add_fscrypt_key() only populates keyid on success,
so this read uninitialized stack memory. Return the error directly
instead, since no key was added and there is nothing to remove.

Also stop overwriting the original error with remove_fscrypt_key()'s
return value on the policy-set failure path, so the real failure is
reported to the caller.

Fixes: https://tracker.ceph.com/issues/78246
Signed-off-by: Sun Yuechi <sunyuechi@iscas.ac.cn>