fanotify: record name info for FAN_DIR_MODIFY event

For FAN_DIR_MODIFY event, allocate a variable size event struct to store
the dir entry name along side the directory file handle.

At this point, name info reporting is not yet implemented, so trying to
set FAN_DIR_MODIFY in mark mask will return -EINVAL.

Link: https://lore.kernel.org/r/20200319151022.31456-14-amir73il@gmail.com


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