fanotify: Use inode_is_open_for_write

Use the aptly named function rather than opencoding i_writecount check.
No functional changes.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>