monitor: Fix for incorrect debug_mask

monitor: Fix for incorrect debug_mask

The field 'debug_mask' of struct netlink_mon_handler was left
uninitialized in do_command_monitor() so it contained garbage from the
stack. Fix this by initializing it with the debug_mask value from struct
netlink_ctx.

While being at it, change the code to make use of C99-style initializer,
which will also avoid things like this in future.

Fixes: be441e1ffdc24 ("src: add debugging mask to context structure")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>