Skip ro check for snaps when multi-mount

Skip ro check for snapshots since they are always ro regardless if ro
flag is passed by mount or not. This allows multi-mounting snapshots
without requiring to specify ro flag.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #16299