fsnotify: Update comments

 *

 * LIFETIME:

 * Inode marks survive between when they are added to an inode and when their

 * refcnt==0.

 * refcnt==0. Marks are also protected by fsnotify_mark_srcu.

 *

 * The inode mark can be cleared for a number of different reasons including:

 * - The inode is unlinked for the last time.  (fsnotify_inode_remove)

 * - The fsnotify_group associated with the mark is going away and all such marks

 *   need to be cleaned up. (fsnotify_clear_marks_by_group)

 *

 * Worst case we are given an inode and need to clean up all the marks on that

 * inode.  We take i_lock and walk the i_fsnotify_marks safely.  For each

 * mark on the list we take a reference (so the mark can't disappear under us).

 * We remove that mark form the inode's list of marks and we add this mark to a

 * private list anchored on the stack using i_free_list; we walk i_free_list

 * and before we destroy the mark we make sure that we dont race with a

 * concurrent destroy_group by getting a ref to the marks group and taking the

 * groups mutex.

 * Very similarly for freeing by group, except we use free_g_list.

 *

 * This has the very interesting property of being able to run concurrently with

 * any (or all) other directions.

 */