kern: never restart syscalls calling closefp(), e.g. close(2)

All paths leading into closefp() will either replace or remove the fd from
the filedesc table, and closefp() will call fo_close methods that can and do
currently sleep without regard for the possibility of an ERESTART. This can
be dangerous in multithreaded applications as another thread could have
opened another file in its place that is subsequently operated on upon
restart.

The following are seemingly the only ones that will pass back ERESTART
in-tree:
- sockets (SO_LINGER)
- fusefs
- nfsclient

Reviewed by:	jilles, kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D27310

sys/kern/kern_descrip.c

@@ -1279,6 +1279,15 @@ closefp(struct filedesc *fdp, int fd, struct file *fp, struct thread *td,
 	FILEDESC_XUNLOCK(fdp);
 
 	error = closef(fp, td);
+
+	/*
+	 * All paths leading up to closefp() will have already removed or
+	 * replaced the fd in the filedesc table, so a restart would not
+	 * operate on the same file.
+	 */
+	if (error == ERESTART)
+		error = EINTR;
+
 	if (holdleaders) {
 		FILEDESC_XLOCK(fdp);
 		fdp->fd_holdleaderscount--;