aesni(4): Properly align the context memory using out kmalloc(9) feature

aesni(4): Properly align the context memory using out kmalloc(9) feature

DragonFly's kmalloc(9) will return 2^n aligned memory if the allocation
size is 2^n; use this feature instead of using a local hack.

While I'm here, release the spinlock and call kmalloc(9) w/ M_WAITOK;
the check/free race is absolutely acceptable here, but the failure
of kmalloc(9) w/ M_NOWAIT is probably not accetable on the init path.