lindebugfs: improve an error message

In case the fill function fails do not report (read/write) but the
actual operation only given we can easily determine it.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	dumbbell, emaste
Differential Revision: https://reviews.freebsd.org/D57523

sys/compat/lindebugfs/lindebugfs.c

@@ -191,8 +191,8 @@ debugfs_fill(PFS_FILL_ARGS)
 
 	if (rc < 0) {
 #ifdef INVARIANTS
-		printf("%s:%d read/write failed with %zd\n", __func__, __LINE__,
-		    rc);
+		printf("%s:%d %s failed with %zd\n", __func__, __LINE__,
+		    (uio->uio_rw == UIO_READ) ? "read" : "write", rc);
 #endif
 		return (-rc);
 	}