fanotify: Wrap object_fh inline space in a creator macro

fanotify_error_event would duplicate this sequence of declarations that
already exist elsewhere with a slight different size.  Create a helper
macro to avoid code duplication.

Link: https://lore.kernel.org/r/20211025192746.66445-23-krisman@collabora.com
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Jan Kara <jack@suse.cz>