proc: Don't use parent proc_self_fd cached descriptor

When we call open_proc(PROC_SELF, ...) the /proc/self descriptor is
cached in criu. If the process fork()-s after than and child goes
open_proc(PROC_SELF, ...) then it will get the parent's proc descriptor.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>

util.c

@@ -174,6 +174,7 @@ int move_img_fd(int *img_fd, int want_fd)
 
 static pid_t open_proc_pid = PROC_NONE;
 static int open_proc_fd = -1;
+static pid_t open_proc_self_pid;
 static int open_proc_self_fd = -1;
 
 static inline void set_proc_self_fd(int fd)
@@ -182,6 +183,7 @@ static inline void set_proc_self_fd(int fd)
 		close(open_proc_self_fd);
 
 	open_proc_self_fd = fd;
+	open_proc_self_pid = getpid();
 }
 
 static inline void set_proc_pid_fd(int pid, int fd)
@@ -195,9 +197,13 @@ static inline void set_proc_pid_fd(int pid, int fd)
 
 static inline int get_proc_fd(int pid)
 {
-	if (pid == PROC_SELF)
+	if (pid == PROC_SELF) {
+		if (open_proc_self_fd != -1 && open_proc_self_pid != getpid()) {
+			close(open_proc_self_fd);
+			open_proc_self_fd = -1;
+		}
 		return open_proc_self_fd;
-	else if (pid == open_proc_pid)
+	} else if (pid == open_proc_pid)
 		return open_proc_fd;
 	else
 		return -1;