Fix eBPF compare-and-exchange instruction

Linux kernel's documentation tells in
https://www.kernel.org/doc/html/v6.0/bpf/instruction-set.html#atomic-operations

> The BPF_CMPXCHG operation atomically compares the value addressed by
> dst_reg + off with R0. If they match, the value addressed by
> dst_reg + off is replaced with src_reg. In either case, the value that
> was at dst_reg + off before the operation is zero-extended and loaded
> back to R0.

If the values don't match, *(dst_reg + off) is not supposed to be
modified.

Fix the semantic of the compare-and-exchange instruction accordingly.

Author: niooss-ledger

Ghidra/Processors/eBPF/data/languages/eBPF.sinc

@@ -371,6 +371,7 @@ DST4: dst is dst { local tmp:4 = dst:4; export tmp; }
     local tmp:4 = *:4 (dst + off);
     if (R0:4 == tmp) goto <equal>;
     R0 = zext(tmp);
+    goto inst_next;
 <equal>
     *:4 (dst + off) = src:4;
 }
@@ -379,6 +380,7 @@ DST4: dst is dst { local tmp:4 = dst:4; export tmp; }
     local tmp:8 = *:8 (dst + off);
     if (R0 == tmp) goto <equal>;
     R0 = tmp;
+    goto inst_next;
 <equal>
     *:8 (dst + off) = src; 
 }