vfs: correctly predict last fdrop on failed open

Arguably since the count is guaranteed to be 1 the code should be modified
to avoid the work.

sys/kern/vfs_syscalls.c

@@ -1229,7 +1229,7 @@ kern_openat(struct thread *td, int fd, const char *path, enum uio_seg pathseg,
 	return (0);
 bad:
 	KASSERT(indx == -1, ("indx=%d, should be -1", indx));
-	fdrop(fp, td);
+	fdrop_close(fp, td);
 	return (error);
 }