coredump: Also dump first pages of non-executable ELF libraries

When I rewrote the VMA dumping logic for coredumps, I changed it to
recognize ELF library mappings based on the file being executable instead
of the mapping having an ELF header. But turns out, distros ship many ELF
libraries as non-executable, so the heuristic goes wrong...

Restore the old behavior where FILTER(ELF_HEADERS) dumps the first page of
any offset-0 readable mapping that starts with the ELF magic.

This fix is technically layer-breaking a bit, because it checks for
something ELF-specific in fs/coredump.c; but since we probably want to
share this between standard ELF and FDPIC ELF anyway, I guess it's fine?
And this also keeps the change small for backporting.

Cc: stable@vger.kernel.org
Fixes: 429a22e7 ("coredump: rework elf/elf_fdpic vma_dump_size() into common helper")
Reported-by: Bill Messmer <wmessmer@microsoft.com>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220126025739.2014888-1-jannh@google.com