elf coredump: make offset calculation process and writing process explicit

By the next patch, elf_core_dump() and elf_fdpic_core_dump() will support
extended numbering and so will produce the corefiles with section header
table in a special case.

The problem is the process of writing a file header offset of the section
header table into e_shoff field of the ELF header.  ELF header is
positioned at the beginning of the corefile, while section header at the
end.  So, we need to take which of the following ways:

 1. Seek backward to retry writing operation for ELF header
    after writing process for a whole part

 2. Make offset calculation process and writing process
    totally sequential

The clause 1.  is not always possible: one cannot assume that file system
supports seek function.  Consider the no_llseek case.

Therefore, this patch adopts the clause 2.

Signed-off-by: Daisuke HATAYAMA <d.hatayama@jp.fujitsu.com>
Cc: "Luck, Tony" <tony.luck@intel.com>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Greg Ungerer <gerg@snapgear.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: <linux-arch@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>