lib: Rename ofp to buf.

dpif-packet contains ofpbuf which points to packet data.  Here buf
is better name rather than ofp.
Following patch renames all remaining instances of ofp variable.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Daniele Di Proietto <ddiproietto@vmware.com>