dm crypt: Slightly simplify crypt_set_keyring_key()

Use strchr() instead of strpbrk() when there is only 1 element in the set
of characters to look for.

This potentially saves a few cycles, but gcc does already account for
optimizing this pattern thanks to it's fold_builtin_strpbrk().
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-crypt.c

@@ -2487,7 +2487,7 @@ static int crypt_set_keyring_key(struct crypt_config *cc, const char *key_string
 	}
 
 	/* look for next ':' separating key_type from key_description */
-	key_desc = strpbrk(key_string, ":");
+	key_desc = strchr(key_string, ':');
 	if (!key_desc || key_desc == key_string || !strlen(key_desc + 1))
 		return -EINVAL;