fsnotify: create helpers to get sb and connp from object

In preparation to passing an object pointer to add/remove/find mark
helpers, create helpers to get sb and connp by object type.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Message-Id: <20240317184154.1200192-3-amir73il@gmail.com>