tailq: use a single cast macro
tailq: use a single cast macro

No need to cast everywhere, define a common macro for this, plus it can be used
in future commits.

Signed-off-by: David Marchand <f28529695cfa9e3e1eb84e7072aa626af4abb18e@6wind.com>
Acked-by: Neil Horman <3316dc2d77df57653443c0391a5296176d6ca9c3@tuxdriver.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk