LU-354 test: Change dev_set_rdonly() check to warning

LU-354 test: Change dev_set_rdonly() check to warning

The dev_set_rdonly() symbol is only required to fence I/O
from the underlying device for testing.  This patch is not
strictly required to build a lustre release.

In addition, this change updates the dt_ro callback to
potentially return an error code.

Change-Id: I35b0a120087845e2a57c9df6c3e1de5b9e4b6ff7
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-on: http://review.whamcloud.com/1872
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>