selftests/fs/mount-notify: test also remove/flush of mntns marks

Regression test for FAN_MARK_MNTFS | FAN_MARK_FLUSH bug.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20250418193903.2607617-3-amir73il@gmail.com