mempool: add namespace to flags

mempool: add namespace to flags

Fix the mempool flags namespace by adding an RTE_ prefix to the name.
The old flags remain usable, to be deprecated in the future.

Flag MEMPOOL_F_NON_IO added in the release is just renamed to have RTE_
prefix.

Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Olivier Matz <olivier.matz@6wind.com>