afs: Fix mounting of backup volumes

In theory the AFS_VLSF_BACKVOL flag for a server in a vldb entry
would indicate the presence of a backup volume on that server.

In practice however, this flag is never set, and the presence of
a backup volume is implied by the entry having AFS_VLF_BACKEXISTS set,
for the server that hosts the read-write volume (has AFS_VLSF_RWVOL).

Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>