Back out read buffer invalidating via __SMOD.

It was correct, but not needed because internal buffer cleared on each seek
outside of it.

lib/libc/stdio/fseek.c

@@ -225,8 +225,8 @@ _fseeko(fp, offset, whence, ltest)
 		goto dumb;
 
 	/*
-	 * If the buffer was modified or the file position was changed,
-	 * we have to skip this; see fgetln.c, stdio.c.
+	 * (If the buffer was modified, we have to
+	 * skip this; see fgetln.c.)
 	 */
 	if (fp->_flags & __SMOD)
 		goto abspos;

lib/libc/stdio/stdio.c

@@ -164,11 +164,6 @@ _sseek(fp, offset, whence)
 	errret = errno;
 	if (errno == 0)
 		errno = serrno;
-
-	if (errret == 0 && (offset != 0 || whence != SEEK_CUR))
-		/* Disallow fseek() optimization inside read buffer */
-		fp->_flags |= __SMOD;
-
 	/*
 	 * Disallow negative seeks per POSIX.
 	 * It is needed here to help upper level caller