fanotify: check FS_ISDIR flag instead of d_is_dir()

All fsnotify hooks set the FS_ISDIR flag for events that happen
on directory victim inodes except for fsnotify_perm().

Add the missing FS_ISDIR flag in fsnotify_perm() hook and let
fanotify_group_event_mask() check the FS_ISDIR flag instead of
checking if path argument is a directory.

This is needed for fanotify support for event types that do not
carry path information.

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