9p: fix bad error path in conversion routines

When buf_check_overflow() returns != 0 we will hit kfree(ERR_PTR(err))
and it will not be happy about it.
Signed-off-by: NMariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: NEric Van Hensbergen <ericvh@gmail.com>

net/9p/conv.c

@@ -796,6 +796,7 @@ struct p9_fcall *p9_create_twrite_u(u32 fid, u64 offset, u32 count,
 	if (err) {
 		kfree(fc);
 		fc = ERR_PTR(err);
+		goto error;
 	}
 
 	if (buf_check_overflow(bufp)) {