ecryptfs: fix string overflow on long cipher names

Passing a cipher name > 32 chars on mount results in an overflow when the
cipher name is printed, because the last character in the struct
ecryptfs_key_tfm's cipher_name string was never zeroed.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Acked-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>