kernfs: don't depend on d_find_any_alias() when generating notifications

kernfs_notify_workfn() sends out file modified events for the
scheduled kernfs_nodes.  Because the modifications aren't from
userland, it doesn't have the matching file struct at hand and can't
use fsnotify_modify().  Instead, it looked up the inode and then used
d_find_any_alias() to find the dentry and used fsnotify_parent() and
fsnotify() directly to generate notifications.

The assumption was that the relevant dentries would have been pinned
if there are listeners, which isn't true as inotify doesn't pin
dentries at all and watching the parent doesn't pin the child dentries
even for dnotify.  This led to, for example, inotify watchers not
getting notifications if the system is under memory pressure and the
matching dentries got reclaimed.  It can also be triggered through
/proc/sys/vm/drop_caches or a remount attempt which involves shrinking
dcache.

fsnotify_parent() only uses the dentry to access the parent inode,
which kernfs can do easily.  Update kernfs_notify_workfn() so that it
uses fsnotify() directly for both the parent and target inodes without
going through d_find_any_alias().  While at it, supply the target file
name to fsnotify() from kernfs_node->name.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Evgeny Vereshchagin <evvers@ya.ru>
Fixes: d911d987 ("kernfs: make kernfs_notify() trigger inotify events too")
Cc: John McCutchan <john@johnmccutchan.com>
Cc: Robert Love <rlove@rlove.org>
Cc: Eric Paris <eparis@parisplace.org>
Cc: stable@vger.kernel.org # v3.16+
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>