To be able to dump procfs namespace entries we will need to
analyze the link name first and then figure out if the file
referred indeed a procfs entry or it's plain regular file.

This means we read link early, and to escape double reading
of same link, once we read it we remember it in fd_parms
structure which we pass to dump_one_reg_file.

Still the dump_one_reg_file is not solely used from inside
of dump_one_file, but from a number of other places (fifo,
special files) where fd_parms is filled only partially
and fill_fd_params is not even called. Thus dump_one_reg_file
must be ready for such scenario and read the link name by own
if the caller has not provided one.

To achieve all this we do

 - extend fd_parms structure to have a reference on a new
   fd_link structure, thus if caller already read the link
   name it might assign the reference and call for dump_one_reg_file

 - tune up dump_one_reg_file to fill own copy of fd_link
   structure if the caller has not provied one

  [ xemul: Added const to fill_fdlink arg ]
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>