MFC 318090,319475: Use const with some read-only buffers in opencrypto APIs.

318090:
Use const with some read-only buffers in opencrypto APIs.

• Mark the source buffer for a copyback operation as const in the kernel API.
• Use const with input-only buffers in crypto ioctl structures used with /dev/crypto.

319475:
Fix some new errors and a warning in cryptotest.

• Use a new 'char *key' to allocate storage for keys and assign the pointer to the session2_op 'const char *' members after the key is initialized.
• Mark the 'find' variable used in crfind() static so that crfind() doesn't return a pointer to stack garbage.

Sponsored by: Chelsio Communications