netlink: specs: clean up spaces in brackets

Clean up all space inside brackets errors reported by yamllint in
the netlink specs:

    error    too many spaces inside brackets  (brackets)

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org> # mptcp_pm.yaml
Link: https://patch.msgid.link/20250610125944.85265-3-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>