libelf: Use GElf_Ehdr instead of Elf handle in __libelf_data_type

libelf: Use GElf_Ehdr instead of Elf handle in __libelf_data_type

GCC with -flto detects some (very) unlikely issues in error paths.
In theory getting the Ehdr from an Elf handle might fail. But in
most cases where __libelf_data_type is used, we already have the
Ehdr anyway. So simply pass that to __libelf_data_type. In the one
place where we don't have it yet, get it and check for failure
before calling __libelf_data_type.

Signed-off-by: Mark Wielaard <mark@klomp.org>