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>