Remove dead code error handling from dsl_crypt.c

Sleepable (KM_SLEEP) allocations cannot fail. Hence
error handling for them is not useful.

Reviewed-By: Tom Caputi <tcaputi@datto.com>
Reviewed-By: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10031