[CIFS] improve setlease handling

fcntl(F_SETLEASE) currently is not exported by cifs (nor by local file
systems) so cifs grants leases based on how other local processes have
opened the file not by whether the file is cacheable (oplocked).  This
adds the check to make sure that the file is cacheable on the client
before checking whether we can grant the lease locally
(generic_setlease).  It also adds a mount option for cifs (locallease)
if the user wants to override this and try to grant leases even
if the server did not grant oplock.

Signed-off-by: Steve French <sfrench@us.ibm.com>