LoongArch: BPF: Add the default case in emit_atomic() and rename it

LoongArch: BPF: Add the default case in emit_atomic() and rename it

Like the other archs such as x86 and riscv, add the default case
in emit_atomic() to print an error message for the invalid opcode
and return -EINVAL, then make its return type as int.

While at it, given that all of the instructions in emit_atomic()
are only read-modify-write instructions, rename emit_atomic() to
emit_atomic_rmw() to make it clear, because there will be a new
function emit_atomic_ld_st() for load-acquire and store-release
instructions in the later patch.

Acked-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>