vfs: Remove mount list trawling cursor stuff

Remove the stuff for trawling a mount namespace's mount list using inserted
cursors as bookmarks as this has been replaced with an xarray-based
approach.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Alexander Viro <viro@zeniv.linux.org.uk>
cc: Miklos Szeredi <miklos@szeredi.hu>
cc: Matthew Wilcox <willy@infradead.org>