[Bug sanitizer/114217] -fsanitize=alignment false positive with intended unaligned struct member access

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114217

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/asm-generic/unaligned.h?h=v6.7

is correct except it should not expose get_unaligned/put_unaligned since the
undefined code happens way before.

The problem is with the btrfs code in btrfs_filldir:
```
static int btrfs_filldir(void *addr, int entries, struct dir_context *ctx)
{
        while (entries--) {
                struct dir_entry *entry = addr; /// THIS IS BROKEN and causes
the -fsanitize=alignment error
                char *name = (char *)(entry + 1);

                ctx->pos = get_unaligned(&entry->offset);
                if (!dir_emit(ctx, name, get_unaligned(&entry->name_len),
                                         get_unaligned(&entry->ino),
                                         get_unaligned(&entry->type)))
                        return 1;
                addr += sizeof(struct dir_entry) +
                        get_unaligned(&entry->name_len);
                ctx->pos++;
        }
        return 0;
}
```

Added comment on where the error comes from. The get_unaligned macro really
should not be used here. What should be used here is an unaligned version of
`struct dir_entry` instead.