dm crypt: fix get_key_size compiler warning if !CONFIG_KEYS

Explicitly convert unsigned int in the right of the conditional
expression to int to match the left side operand and the return type,
fixing the following compiler warning:

drivers/md/dm-crypt.c:2593:43: warning: signed and unsigned
type in conditional expression [-Wsign-compare]

Fixes: c538f6e ("dm crypt: add ability to use keys from the kernel key retention service")
Signed-off-by: Aashish Sharma <shraash@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-crypt.c

@@ -2582,7 +2582,7 @@ static int crypt_set_keyring_key(struct crypt_config *cc, const char *key_string
 
 static int get_key_size(char **key_string)
 {
-	return (*key_string[0] == ':') ? -EINVAL : strlen(*key_string) >> 1;
+	return (*key_string[0] == ':') ? -EINVAL : (int)(strlen(*key_string) >> 1);
 }
 
 #endif /* CONFIG_KEYS */