Fix bug #5460. The problem is RHEL5.0 shipped a CIFS client

Fix bug #5460. The problem is RHEL5.0 shipped a CIFS client
that sets the DFS bit on pathnames but doesn't
send DFS paths. This causes lookups to fail as
the smbd/msdfs.c code now just eats the first
two parts of the pathname and uses the rest as
the local path. The previous hostname check
used to protect us from that as we knew that
when the hostname was invalid it was a local
path (and a broken client).
I didn't want to put that check back in, but
came up with another idea - even though the
hostname can be a different one, the sharename
must be valid on this machine. So we can check
for a valid sharename instead.
Jeremy.
(This used to be commit e1cda82f6f7de3306a653af920756c1640057f2d)