fsnotify: fsnotify marks on inodes pin them in core

This patch pins any inodes with an fsnotify mark in core.  The idea is that
as soon as the mark is removed from the inode->fsnotify_mark_entries list
the inode will be iput.  In reality is doesn't quite work exactly this way.
The igrab will happen when the mark is added to an inode, but the iput will
happen when the inode pointer is NULL'd inside the mark.

It's possible that 2 racing things will try to remove the mark from
different directions.  One may try to remove the mark because of an
explicit request and one might try to remove it because the inode was
deleted.  It's possible that the removal because of inode deletion will
remove the mark from the inode's list, but the removal by explicit request
will actually set entry->inode == NULL; and call the iput.  This is safe.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@lst.de>