Extend usr/gen_init_cpio.c "file" entry, adding support for hard links.

Previous format:
file <name> <location> <mode> <uid> <gid>

New format:
file <name> <location> <mode> <uid> <gid> [<hard links>]

The hard links specification is optional, keeping the previous
behaviour.

All hard links are defined sequentially in the resulting cpio and the
file data is present only in the last link. This is the behaviour of
GNU's cpio and is supported by the kernel initramfs extractor.
Signed-off-by: Luciano Rocha <strange@nsk.no-ip.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>