eal/linux: remove useless casts in hugepage init

Rather than cast the huge pages number returned by get_num_hugepages, rework
this function so that it returns 0 when something goes wrong.
And no need for casts in log.

Signed-off-by: David Marchand <david.marchand@6wind.com>
Acked-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>