memtable: fix gcc function argument evaluation order induced use after move
memtable: fix gcc function argument evaluation order induced use after move

clang evaluates function arguments from left to right, while gcc does so
in reverse. Therefore, this code can be correct on clang and incorrect
on gcc:
```
f(x.sth(), std::move(x))
```

This patch fixes one such instance of this bug, in memtable.cc.

Fixes #9605.

Closes #9606

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla