pdump: fix race in disabling

pdump: fix race in disabling

There is a race where the request to disable pdump may get ahead
of the handling of pdump requests in dumpcap. The fix is to do
local removal of callbacks before forwarding same to secondary.

To reproduce:
   1. Start testpmd and start traffic
   2. Start dumpcap to capture
   3. Interrupt dumpcap with ^C

Testpmd will show missing response and dumpcap will show error:
EAL: Cannot find action: mp_pdump

Only reproducible if additional logging not enabled.

Fixes: c3ceb8742295 ("pdump: forward callback enable to secondary process")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>