proc_parse: parse_smaps -- Use @file_path instead of strstr helper

strstr is a really heavy one, lets use already defined
and filled @file_path variable instead.

Reported-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>