mempool: fix anonymous populate

The documentation says that a negative errno is returned on error, but
in most places that's not the case.

Fix the documentation and the exceptions in code. The second one
(return from populate_virt) also fixes a memory leak.

Note that testpmd was using the function correctly.

Fixes: aa10457eb4c2 ("mempool: make mempool populate and free api public")
Fixes: 6780f72fb82f ("mempool: populate with anonymous memory")
Fixes: 66e7ba0bad4c ("mempool: ensure mempool is initialized before populating")
Cc: stable@dpdk.org
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>