nfs: propagate readlink errors in nfs_symlink_filler

There is an inherent race where a symlink file may have been overriden
(by a different client) between lookup and readlink, resulting in a
spurious EIO error returned to userspace. Fix this by propagating back
ESTALE errors such that the vfs will retry the lookup/get_link (similar
to nfs4_file_open) at least once.

Cc: Dan Aloni <dan.aloni@vastdata.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/symlink.c

@@ -41,7 +41,7 @@ static int nfs_symlink_filler(struct file *file, struct folio *folio)
 error:
 	folio_set_error(folio);
 	folio_unlock(folio);
-	return -EIO;
+	return error;
 }
 
 static const char *nfs_get_link(struct dentry *dentry,