vfs: Use IOP_XATTR flag for bad-inode handling

With this change, all the xattr handler based operations will produce an
-EIO result for bad inodes, and we no longer only depend on inode->i_op
to be set to bad_inode_ops.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>