staging: lustre: check for integer overflow

In ll_ioctl_fiemap(), a user-supplied value is used to calculate a
length of a buffer which is later allocated with user data.

Signed-off-by: Vitaly Osipov <vitaly.osipov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>