coredump: Use the vma snapshot in fill_files_note

Matthew Wilcox reported that there is a missing mmap_lock in
file_files_note that could possibly lead to a user after free.

Solve this by using the existing vma snapshot for consistency
and to avoid the need to take the mmap_lock anywhere in the
coredump code except for dump_vma_snapshot.

Update the dump_vma_snapshot to capture vm_pgoff and vm_file
that are neeeded by fill_files_note.

Add free_vma_snapshot to free the captured values of vm_file.

Reported-by: Matthew Wilcox <willy@infradead.org>
Link: https://lkml.kernel.org/r/20220131153740.2396974-1-willy@infradead.org
Cc: stable@vger.kernel.org
Fixes: a07279c9a8cd ("binfmt_elf, binfmt_elf_fdpic: use a VMA list snapshot")
Fixes: 2aa362c49c31 ("coredump: extend core dump note section to contain file names of mapped files")
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
