LU-15451 sec: retry ro mount if read-only flag set

LU-15451 sec: retry ro mount if read-only flag set

In case client mount fails with -EROFS because the read-only nodemap
flag is set and ro mount option is not specified, just retry ro mount
internally. This is to avoid the need for users to manually retry the
mount with ro option.

Lustre-change: https://review.whamcloud.com/47490
Lustre-commit: TBD (db0765ffda943399c9c095fbf82bcab9079feadd)

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I0dedd1394eeb6804f7fdde930275f6649b935bab
Reviewed-on: https://review.whamcloud.com/47817
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>