Extend the comment about checking for a race with close to explain why

it is done and why we don't return an error in such case.

Discussed with:	kib
MFC after:	1 month

sys/kern/kern_descrip.c

@@ -672,7 +672,23 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 			fdrop(fp, td);
 			break;
 		}
-		/* Check for race with close */
+
+		/*
+		 * Check for a race with close.
+		 *
+		 * The vnode is now advisory locked (or unlocked, but this case
+		 * is not really important) as the caller requested.
+		 * We had to drop the filedesc lock, so we need to recheck if
+		 * the descriptor is still valid, because if it was closed
+		 * in the meantime we need to remove advisory lock from the
+		 * vnode - close on any descriptor leading to an advisory
+		 * locked vnode, removes that lock.
+		 * We will return 0 on purpose in that case, as the result of
+		 * successful advisory lock might have been externally visible
+		 * already. This is fine - effectively we pretend to the caller
+		 * that the closing thread was a bit slower and that the
+		 * advisory lock succeeded before the close.
+		 */
 		FILEDESC_SLOCK(fdp);
 		if (fget_locked(fdp, fd) != fp) {
 			FILEDESC_SUNLOCK(fdp);