For private namespaces ns->head_disk is NULL, so add a NULL check
before updating the BDI capabilities.

Fixes: b2ce4d90 ("nvme-multipath: set bdi capabilities once")
Reported-by: NAvinash M N <Avinash.M.N@wdc.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NSagi Grimberg <sagi@grimberg.me>
Reviewed-by: NMax Gurtovoy <maxg@mellanox.com>