Clean up crypto_init().

The function is called from a KLD load handler, so it may sleep.

• Stop checking for errors from uma_zcreate(), they don't happen.
• Convert M_NOWAIT allocations to M_WAITOK.
• Remove error handling for existing M_WAITOK allocations.
• Fix style.

Reviewed by: cem, delphij, jhb
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25696