r15668: DOS or FCB opens share one share mode entry from different

r15668: DOS or FCB opens share one share mode entry from different
fsp pointers. Ensure we cope with this to pass Samba4
DENY tests (we used to pass these, there must have been
a regression with newer code). We now pass them.
Jeremy
(This used to be commit fd6fa1d4eaf61783df74ee2da50d331477f06998)