afs: Fix bulk lookup malfunction due to change in dir_emit() API

afs_do_lookup() and afs_do_lookup_one() use the same directory parsing code
as afs_readdir() and were supplying alternative dir_context actors to
retrieve dirents, but because lookup needs the vnode's uniquifier as part
of the reference, but not the DT flags, the uniquifier was being passed in
the dt flags argument to the lookup actors.

Unfortunately, commit c644bce62b9c, added to fix overlayfs with fuse, broke
this by masking off part of the uniquifier.  This doesn't matter enough to
be directly noticeable, instead causing bulk advance inode lookups to fail
(which are retried later) and may cause dir revalidation to malfunction if
the uniquifier is changed by masking.

Fix this by making the afs directory parsing code take special ->actor
values of AFS_LOOKUP or AFS_LOOKUP_ONE instead that tell it to call
afs_lookup_filldir() or afs_lookup_one_filldir() directly rather than going
through dir_emit().  dir_emit() is still used for readdir.

Fixes: c644bce62b9c ("readdir: require opt-in for d_type flags")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Amir Goldstein <amir73il@gmail.com>
cc: linux-afs@lists.infradead.org