UPSTREAM: kernfs: switch kernfs to use an rwsem

The kernfs global lock restricts the ability to perform kernfs node
lookup operations in parallel during path walks.

Change the kernfs mutex to an rwsem so that, when opportunity arises,
node searches can be done in parallel with path walk lookups.

Reviewed-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Link: https://lore.kernel.org/r/162642770946.63632.2218304587223241374.stgit@web.messagingengine.com


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

(cherry picked from commit 7ba0273b)

Bug: 320903885
Bug: 219424218
Bug: 206126556
Change-Id: I88ff2dfeef30a0af6a0b8c708f0f505b1a347637
Signed-off-by: Suren Baghdasaryan <surenb@google.com>