crypto: cryptd - simplify error handling in cryptd_create_*()

Simplify the error handling in the various cryptd_create_*() functions
by taking advantage of crypto_grab_*() now handling an ERR_PTR() name
and by taking advantage of crypto_drop_*() now accepting (as a no-op) a
spawn that hasn't been grabbed yet.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>