fdcopy(): properly update fd_freefile when skipping non-copyable fd

fd_freefile is maintained to fulfill the POSIX guarantee that the lowest
possible file descriptor is allocated on open.  If we skip copying an
fd, it is a possible candidate for the lowest index.

First, the file descriptor installed with a kqueue created
without KQUEUE_CPONFORK missed this handling [*].

Second, fd_freefile should be updated only when the current fd index is
below it.  A skipped fd above fd_freefile could raise it past a free slot.

PR:	300234 [*]
Reported by:	Andrea Venturoli <[email protected]>
Reviewed by:	markj
Fixes:	1265516c5c ("struct file: add DFLAG_FORK, indicate copying on fork")
Fixes:	809f3121be ("fd: assign fd_freefile early when copying")
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D61054

sys/kern/kern_descrip.c

@@ -2644,13 +2644,16 @@ fdcopy(struct filedesc *fdp, struct proc *p1)
 		if ((ops->fo_flags & DFLAG_FORK) != 0 &&
 		    (ofde->fde_flags & UF_FOCLOSE) == 0) {
 			if (ops->fo_fork(newfdp, ofde->fde_file, &fp, p1,
-			    curthread) != 0)
+			    curthread) != 0) {
+				if (newfdp->fd_freefile > i)
+					newfdp->fd_freefile = i;
 				continue;
+			}
 			fork_pass = true;
 		} else if ((ops->fo_flags & DFLAG_PASSABLE) == 0 ||
 		    (ofde->fde_flags & UF_FOCLOSE) != 0 ||
 		    !fhold(ofde->fde_file)) {
-			if (newfdp->fd_freefile == fdp->fd_freefile)
+			if (newfdp->fd_freefile > i)
 				newfdp->fd_freefile = i;
 			continue;
 		}