ext2fs: fix -Walloc-size

ext2fs: fix -Walloc-size

GCC 14 introduces a new -Walloc-size included in -Wextra which gives:
```
lib/ext2fs/hashmap.c:37:36: warning: allocation of insufficient size ‘1’ for type ‘struct ext2fs_hashmap’ with size ‘20’ [-Walloc-size]
```

The calloc prototype is:
```
void *calloc(size_t nmemb, size_t size);
```

So, just swap the number of members and size arguments to match the prototype, as
we're initialising 1 struct of size `sizeof(...)`. GCC then sees we're not
doing anything wrong.

Signed-off-by: Sam James <sam@gentoo.org>
Link: https://lore.kernel.org/r/20231107233122.2013191-1-sam@gentoo.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>