pg_upgrade: fix memory leak in SLRU I/O code

Hi Hackers,

This comes from a previous review and has been on my to-do list for a while.

Since src/bin/pg_upgrade/slru_io.c includes postgres_fe.h, it is frontend code, so backend memory contexts are not used
here.

In the current code:
```
void
FreeSlruWrite(SlruSegState *state)
{
    Assert(state->writing);

    SlruFlush(state);

    if (state->fd != -1)
        close(state->fd);
    pg_free(state);
}
```

the SlruSegState itself is freed, but state->dir and state->fn are not, which results in a memory leak during
pg_upgraderuns. More generally, I don’t see a reason to free an object itself without also freeing the memory owned by
itsmembers. 

While looking at this, I also noticed that state->dir is allocated using pstrdup(). To better align with frontend
conventions,the patch switches this to pg_strdup() and introduces a common cleanup helper to free all resources
associatedwith SlruSegState. 

See the attached patch.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/