afs: Fix mmap

Fix afs_add_open_map() to check that the vnode isn't already on the list
when it adds it.  It's possible that afs_drop_open_mmap() decremented
the cb_nr_mmap counter, but hadn't yet got into the locked section to
remove it.

Also vnode->cb_mmap_link should be initialised, so fix that too.

Fixes: 6e0e99d5 ("afs: Fix mmap coherency vs 3rd-party changes")
Reported-by:  <kafs-testing+fedora34_64checkkafs-build-300@auristor.com>
Suggested-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by:  <kafs-testing+fedora34_64checkkafs-build-300@auristor.com>
cc: linux-afs@lists.infradead.org
Link: https://lore.kernel.org/r/686465.1639435380@warthog.procyon.org.uk/

 # v1
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>