staging: lustre: validate size in ll_setxattr()

If size is smaller than the lov_user_md struct then we are reading
beyond the end of the buffer.  I guess this is an information leak or it
could cause an Oops if the memory is not mapped.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>