bpf: add more checks

bpf: add more checks

Add checks for:
 - all instructions are valid ones
   (known opcodes, correct syntax, valid reg/off/imm values, etc.)
 - no unreachable instructions
 - no loops
 - basic stack boundaries checks
 - division by zero

Still need to add checks for:
 - use/return only initialized registers and stack data.
 - memory boundaries violation

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>