Missing atomic instruction tests with BPF_FETCH

[dthaler]

Currently only about half of the atomic instructions have tests. It is missing ones with BPF_FETCH.
https://github.com/dthaler/ebpf-docs/blob/update/isa/kernel.org/instruction-set.rst#atomic-operations

[Alan-Jowett]

Proposal is to extend the lock mnemonic with an optional fetch keyword:

lock [fetch] add register_and_offset, target_register

With the fetch keyword being optional.

If this is agreeable, I will extend the BPF assembler to support these mnemonics.

[dthaler]

That is ok, but you also need to support BPF_XCHG and BPF_CMPXCHG, which are new instructions that only work with BPF_FETCH.