pseudofs: Ignore unsupported commands in vop_setattr.

Users of pseudofs (e.g. lindebugfs), should be able to receive
input from command line via commands like "echo 1 > /path/to/file".
Currently this fails because sh tries to truncate the file first and
vop_setattr returns not supported error for this. This patch simply
ignores the error and returns 0 instead.

Reviewed by:	imp (mentor), asomers
Approved by:	imp (mentor), asomers
MFC after:	1 week
Differential Revision: D20451

sys/fs/pseudofs/pseudofs_vnops.c

@@ -967,7 +967,8 @@ pfs_setattr(struct vop_setattr_args *va)
 	PFS_TRACE(("%s", pn->pn_name));
 	pfs_assert_not_owned(pn);
 
-	PFS_RETURN (EOPNOTSUPP);
+	/* Silently ignore unchangeable attributes. */
+	PFS_RETURN (0);
 }
 
 /*