eCryptfs: change the type of cipher_code from u16 to u8

Only the lower byte of cipher_code is ever used, so it makes sense
for its type to be u8.

Signed-off-by: Trevor Highland <trevor.highland@gmail.com>
Cc: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>