seq_file: add a field to reference the file->f_cred

Export the file->f_cred to the seq_files. This field will be used to
check the cred of process at open

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
seq_file: set the f_cred of the seq_file at open

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Procfs: add the ptrace check at open time for ONE files

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>