[CIFS] Provide sane values for nlink

Since we don't get info about the number of links from the readdir
linfo levels, stat() will return 0 for st_nlink, and in particular,
samba re-exported shares will show directories as files (as samba is
keying off st_nlink before evaluating how to set the dos modebits)
when doing a dir or ls.

Copy nlink to the inode, unless it wasn't provided.  Provide
sane values if we don't have an existing one and none was provided.

Signed-off-by: Jim McDonough <jmcd@samba.org>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: David Disseldorp <ddiss@samba.org>
Signed-off-by: Steve French <smfrench@gmail.com>