9p: fix O_APPEND in legacy mode

The legacy protocol's open operation doesn't handle an append operation
(it is expected that the client take care of it).  We were incorrectly
passing the extended protocol's flag through even in legacy mode.  This
was reported in bugzilla report #10689.  This patch fixes the problem
by disallowing extended protocol open modes from being passed in legacy
mode and implemented append functionality on the client side by adding
a seek after the open.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>
diff --git a/fs/9p/v9fs_vfs.h b/fs/9p/v9fs_vfs.h
index fd01d90..57997fa 100644
--- a/fs/9p/v9fs_vfs.h
+++ b/fs/9p/v9fs_vfs.h
@@ -51,4 +51,4 @@
 int v9fs_file_open(struct inode *inode, struct file *file);
 void v9fs_inode2stat(struct inode *inode, struct p9_stat *stat);
 void v9fs_dentry_release(struct dentry *);
-int v9fs_uflags2omode(int uflags);
+int v9fs_uflags2omode(int uflags, int extended);