POSIX extensions disabled on client due to illegal O_EXCL flag sent to Samba

Samba rejected libreoffice's attempt to open a file with illegal
O_EXCL (without O_CREAT).  Mask this flag off (as the local
linux file system case does) for this case, so that we
don't have disable Unix Extensions unnecessarily due to
the Samba error (Samba server is also being fixed).

See https://bugzilla.samba.org/show_bug.cgi?id=9519



Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>