Reserve OBD_CONNECT2_ENC connection flag so that 'encrypt' or
'test_dummy_encryption' client mount options can only be used if
server side knows how to handle encrypted object size properly.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I42d0b597df3b68bd1de19394104e7fda1b76bf6c
Reviewed-on: https://review.whamcloud.com/36360
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>