LU-10092 pcc: Reserve a new connection flag for PCC

Reserve OBD_CONNECT2_PCC connection flag that will be set
(in ocd_connect_flags2) if a Lustre server or a client supports
Persistent Client Cache (PCC).

Test-Parameters: trivial
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: Ibe20c668a649be69475dc326ce56dc8708772d32
Reviewed-on: https://review.whamcloud.com/34356
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>