mbuf: add namespace to offload flags

Fix the mbuf offload flags namespace by adding an RTE_ prefix to the
name. The old flags remain usable, but a deprecation warning is issued
at compilation.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Acked-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Acked-by: Somnath Kotur <somnath.kotur@broadcom.com>