Support separate output buffers for aesni(4).

The backend routines aesni(4) call for specific encryption modes all
expect virtually contiguous input/output buffers. If the existing
output buffer is virtually contiguous, always write to the output
buffer directly from the mode-specific routines. If the output buffer
is not contiguous, then a temporary buffer is allocated whose output
is then copied to the output buffer. If the input buffer is not
contiguous, then the existing buffer used to hold the input is also
used to hold temporary output.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24545