lpm: fix allocation of an existing object

lpm: fix allocation of an existing object

Fix rte_lpm_create_*() functions to return NULL and set rte_errno to
EEXIST when lpm object name already exists.
This is the behavior described in the API documentation in the header
file.

Fixes: 134975073af3 ("lib: remove unnecessary pointer cast")

Signed-off-by: Pavan Nikhilesh <pbhagavatula@caviumnetworks.com>
Acked-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Ferruh Yigit <ferruh.yigit@intel.com>