Port LL_ALLOCATED_SPACE from 2.6.26

2.6.26 added:

       ndev->needed_headroom
       ndev->needed_tailroom

Then commit SHA1 f5184d267c1aedb9b7a8cc44e08ff6b8d382c3b5
added LL_ALLOCATED_SPACE and made most users of
LL_RESERVED_SPACE make use LL_ALLOCATED_SPACE instead.
Where needed_headroom and needed_tailroom is set we
let the old kernels through as they still are using
LL_ALLOCATED_SPACE

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>