[rtsan] Fix comment in fcntl, fix va_args type (#108440)
Two changes here:
1. Add significantly more detail on why this is OK, from the
conversation here:
https://discourse.llvm.org/t/how-to-write-an-interceptor-for-fcntl/81203
2. Change the type we expect from va_args to intptr_t, which was also a
suggestion in that thread.
