btrfs: return -ENOMEM on allocation failure in btrfsic

Forward the correct return value -ENOMEM from btrfsic_dev_state_alloc()
too.

Signed-off-by: Allen Pais <allen.lkml@gmail.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ adjust changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>