UPSTREAM: kernfs: use VFS negative dentry caching

If there are many lookups for non-existent paths these negative lookups
can lead to a lot of overhead during path walks.

The VFS allows dentries to be created as negative and hashed, and caches
them so they can be used to reduce the fairly high overhead alloc/free
cycle that occurs during these lookups.

Use the kernfs node parent revision to identify if a change has been
made to the containing directory so that the negative dentry can be
discarded and the lookup redone.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Link: https://lore.kernel.org/r/162642770420.63632.15791924970508867106.stgit@web.messagingengine.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(cherry picked from commit c7e7c042)

Bug: 320903885
Bug: 219424218
Bug: 206126556
Change-Id: I1881be14cdca89bd7753ece24daf491299e196ad
Signed-off-by: Suren Baghdasaryan <surenb@google.com>