fs: ecryptfs: Use crypto_wait_req

This patch replaces the custom crypto completion function with
crypto_req_done.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>